A 55-year-old man, a social drinker and a technical officer by profession presented to the Teaching Hospital, Peradeniya (THP) in the Central Province of Sri Lanka with a history of headache, gradual alteration of level of consciousness (LOC) and behavioral changes of 3 weeks duration. He was apparently well 7 weeks back and experienced headache, faintishness and vomiting at 2 am while doing a night shift in his work place. After the patient has lost consciousness and become unresponsive. The following day morning, other workers who came for the morning shift found him drowsy and less responsive and took him to the nearest hospital after 6 hours of initial symptoms. On admission, his Glasgow Coma Scale (GCS) was 9/15 and there were no any focal neurological signs. He was haemodynamically stable and respiratory examination was normal with normal pulse- oximeter finding. He continued to have vomiting and diarrhoea over the next 48 h. His full blood count, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), liver and renal profiles were normal. He was treated with 6 L of oxygen via a face mask and was managed as food poisoning. He was gradually improving and was discharged from the hospital on the 4th day of admission and he was totally normal on discharge. After discharge he was asymptomatic and was doing day to day activities normally. After 1 month of initial episodes, he developed reduced self-care, a lack of interest of work in which he was interested before the illness. He found it difficult to work as a technical officer due to low energy, lethargy, poor concentration and reduced memory. He was withdrawn at times and was less communicative. He endorsed poor sleep and had a low mood. His condition deteriorated over the next 2 weeks and developed episodic inappropriate talking, confusion, generalized rigidity of the body and urinary and fecal incontinence. He was unable to identify his family members and developed reduced level of consciousness. He was admitted to the THP 7 weeks after the initial incidence. On admission to the THP, he was haemodynamically stable with blood pressure of 140/90. On neurological examination, he was drowsy with GCS of 11/12 (E3, V3, M5). Pupils were normal in size and well reacting to light, both optic fundi were normal, and all cranial nerves were normal. He had generalized rigidity with hyperreflexia and bilateral extensor plantar response. He gradually developed masklike face, positive glabella sign and primitive reflexes (grasp reflex). Mini-Mental State Examination (MMSE) was unable to take because of his demented status. His full blood count, blood picture, ESR, CRP, serum magnesium and calcium levels, liver and renal profiles were normal. He had normal electrocardiogram with normal 2D echocardiogram and troponin I was negative. His EEG showed marked generalized slowing of background with multifocal high amplitude waves and delta waves suggestive gross cerebral dysfunction without epileptiform discharges. Non contrast computer tomography (CT) of brain showed prominent cerebral white matter with obliterated sulci with preserved gray-white demarcation and normal ventricular system with no evidence of intra cerebral haemorrhage. His cerebrospinal fluid (CSF) report was normal and CSF was negative for both bacterial studies (light microscopy examination and culture) and viral studies (HSV/Entero/Flavi). The MRI brain showed diffuse high signal intensity involving subcortical white matter, globus pallidus on FLAIR and T2W images. These areas showed high signals in DWI images with no significant changes appreciated on ADC map. There was no abnormal contrast enhancement appreciated in the above areas. Rest of the cerebral hemisphere, ventricular system, brain stem and cerebellum were normal. While investigating this patient (index case) we came to know that another person (2nd case), who did a night duty with our patient and slept in the same room, also had been admitted with our patient to the same local hospital with the initial incidence. He (2nd case) too has experienced vomiting, diarrhoea, faintishness and unsteady gait, but no changing sensorium. On admission to local hospital his (2nd case) unsteady gait improved, and he had vomiting and few more episodes of diarrhoea. He was totally normal on next day and was discharged from the hospital with a diagnosis of food poisoning. Since then he was asymptomatic. Further inquiry revealed an another incidence happened in the same place after 2 weeks of initial incidence when 8 workers who came from their main office (Colombo) slept in the same room and all of them became ill after being there for about 3 hours. The most common symptoms were headache and faintishness which were experienced by all 8 workers. Six of them had unsteady gait and four of them fallen on ground when they attempted to walk. Five of them experienced lethargy and nausea and two had vomiting. Loss of consciousness was experienced by three patients and duration of loss of consciousness (LOC) was less than 5 minutes in two of them and around 20 min for the other one. Even though, it was impossible to recall the associated seizure by others, one had urinary and fecal incontinence at the time of LOC. Three people had diarrhoea and two experienced burning sensation of throat. All of them were taken out and two became totally normal within 30 min. Another two became normal within 2 hours. Two patients out of three who developed LOC became asymptomatic after 12 h and they were treated with face mask oxygen and other supportive care at the same local hospital. All the symptoms except headache experienced by the person who had LOC of around 20 min settled after 24 h and his headache lasted for 3 days. The remaining patient had headache for 3 days even though his initial symptoms were mild. All of them were discharged from the local hospital on the same day evening. After this incidence, the work place was examined by the authorities and found to have newly implanted petrol driven generator, in the ground floor of the two-story building in which all victims (patients) slept. It was found that mal functioning generator emitting carbon monoxide when it was on. In both incidences, there had been a power failure requiring power to be generated by this offending generator. Subsequently, we traced all exposed victims and examined them, including the person who developed symptoms in initial incidence with our patient (index case), after 8 weeks of initial exposure. All of them were neurologically normal and all had normal MRI brain imaging. Depending on the available history, examination, epidemiology of the incidence and the MRI brain finding, the diagnosis of Carbon Monoxide-induced Delayed Neuropsychiatric Sequelae was made. Supportive management was offered to him. He was started on Sinemet (carbidopa levodopa) up to maximum dosages with no clear therapeutic benefit. Dexamethasone was also tried without obvious effect. He was gradually deteriorated over next 2 weeks, exhibited athetoid movements of his feet, and hands and went to rigid akinetic mute state. He could not respond to any stimulation and even displayed decorticate-like posture. His illness was complicated with aspiration pneumonia and died nearly after 3 weeks of admission. His post mortem examination showed evidence of pneumonia mainly in right lung and punctuate haemorrages in the sub cortical white matter of the brain. Histological examinations of all the organs were normal including brain. All the others who had exposure history were followed up for 1 year and they were totally asymptomatic at 1 year of initial incidence.