The 10-year-old boy described here (Full Intelligence Quotient [FIQ] = 112, Verbal Intelligence Quotient [VIQ] = 106, Performance Intelligence Quotient [PIQ] = 117) had been diagnosed with a developmental delay in head control, speech, and language by a paediatrician when he was 1 year and 6 months old. When he entered kindergarten, he often played by himself and did not make friends because of his communication problems. After entering the local elementary school, at the age of 6, he began to show hyperactivity and impulsivity. In addition, he displayed symptoms of motor and vocal tics. He was assessed at a local clinic, and diagnosed with ADHD/ASD comorbid with TS. Although he initially continued to take risperidone (0.5 mg/day), side effects such as headache and anxiety led him to discontinue the treatment. When he was 9 years old, worsened impulsivity led him to behave violently toward his mother. He, therefore, began treatment, at a local clinic, with atomoxetine (ATX) (30 mg/day). However, he discontinued the medication as he experienced worsening irritability. Although he was prescribed MPH (18 mg/day) after discontinuing the treatment with ATX, he also discontinued taking MPH, because his motor and vocal tic symptoms were exacerbated. As these symptoms continued, he was referred to our hospital at 10 years of age, with an ADHD-RS-IV-J score of 23 and a YGTSS score of 29. According to his father, he had only few friends because he had so little interest in making friends in school. The teachers often reported problems to his parents, such as when he showed physical aggression toward his friends or ran away from school during the lesson. He often got angry when the timing of activities deviated from his usual schedule. He was, therefore, diagnosed with ADHD/ASD/TS according to the criteria specified in the DSM-5. The patient was continuously prescribed GUAN at a dose starting at 1 mg/day and increasing to 3 mg/day. However, the 3-mg dose led to drowsiness (ADHD-RS-IV-J score of 10, YGTSS score of 15), and was, therefore, reduced again to 2 mg/day. At the decreased dose, he continued to take GUAN without side effects (ADHD-RS-IV-J score of 9, YGTSS score of 15), while there was no clear difference in effect between the 2-mg/day and the 3-mg/day doses. Importantly, his ADHD-related symptoms, such as irritability, hyperactivity, and inattention, as well as his tic symptoms, gradually improved. On the other hand, GUAN had no effect on ASD symptoms in this case. The patient was able to continue taking GUAN for 6 more months (ADHD-RS-IV-J score of 9, YGTSS score of 15).