We describe the case of a 20-year-old man with schizophrenia who was treated with the dibenzothiazepine antipsychotic drug quetiapine in oral doses of 900 mg daily. At admission the patient had a normal blood count. 15 weeks after initiation of treatment asymptomatic cytopenia was discovered with leucocytes of 2.2 billion/l and thrombocytes of 114 billion/l. Two weeks after discontinuation of quetiapine the leucopenia subsided. Physicians should be aware of leucopenia as a potential adverse effect to quetiapine, especially in patients with symptoms of infections or cytopenia.