A 42-year-old female with a severe headache and Glasgow Coma Scale (GCS) of 15 was admitted from a regional hospital close to our neurosurgical intensive care unit. Digital subtraction angiography showed a ruptured, right-sided ICA aneurysm near the bifurcation followed by a subarachnoid haemorrhage (Fisher III, World Federation of Neurosurgical Societies I: GCS 15, no motoric deficit). A neurological examination revealed no deficits. The medical history of the patient included obesity (a body mass index of 29), Hashimoto thyroiditis, and an erythrocytosis (Hb 17.5 g/dl), in addition to the HbY. The diagnosis of HbY was initially performed at the University of Ulm (Germany) in 2002 through molecular genetic analysis, subsequent to the exclusion of alternative explanations for erythrocytosis. Preoperative laboratory findings showed a haematocrit of 54%, Hb 17.5 g/dl, reticulocytes 25/1000 and erythrocytes 6.2 x 106/μl. After induction of anaesthesia with propofol (2 mg/kg), remifentanyl (0.5 μg/kg/min) and rocuronium (70 mg), an arterial and a central-venous catheter were placed in addition to two peripheral intravenous 17 G catheters. The lungs were ventilated with a fraction of inspired oxygen (FiO2) of 0.5 and a minute ventilation (MV) of 110 ml/min/kg. Nimodipine was continually administered at a rate of 1 mg/h. Anaesthesia was maintained with Propofol (8 mg/kg/h) and Remifentanyl (0.25 μg/kg/h). The operation was started and progressed uneventfully until the surgeon tried to dissect the basis of the aneurysm, which led to severe bleeding. Several temporary clips were required to stop the haemorrhage (approx. 1600 ml). Subsequent to volume resuscitation with 1000 ml Ringer's solution and 500 ml of volume expander (hydroxyethyl starch; HAES) 130/0.4, the haemoglobin value was 10.8 g/dl. During the bleeding interval, no S-T segment changes were observed. However, typical signs of rapid fluid loss, such as a moderately increased heart rate and a temporary drop in blood pressure, were present. Systolic blood pressure was always kept between 120 mmHg and 140 mmHg using noradrenalin in varying dosages (max. 0.25 μg/kg/min). Meanwhile, the patient received 6 units of packed red blood cells (PRBC) and further fluid substitutions. Further bleeding was stopped by continuous compression with TachoSil®, enabling the obliteration of the aneurysm. No vascular occlusions were detected on an indocyanine green angiography. The blood pressure was subsequently pharmacologically raised to a systolic value of approximately 140 mmHg, with no further detectable haemorrhaging. It was possible to extubate the patient within a few minutes upon completion of the operation. Immediate neurological assessment revealed no detectable deficits. The patient was transferred to the intensive care unit with haemoglobin of 16.8 g/dl. Further recovery was uneventful.