In the examination in question, the case history of a patient who had exhibited serious obsessive-compulsive symptoms since the age of 14 is described. It is not an independent obsessive-compulsive disorder, but a case of schizophrenia masked by obsessive-compulsive symptoms. During therapy for the schizophrenic symptoms with clozapine, which set in at the age of 31, neither psychotic symptoms nor obsessive-compulsive symptoms worth mentioning can be found at the 1-year catamnesis. Whereas the literature increasingly points to obsessive-compulsive symptoms being a side-effect of clozapine medication, usually in the clinical indication area of chronic schizophrenic psychoses, as far as the author knows, this describes a rare case of improvement of obsessive-compulsive symptoms using clozapine after first suffering from a schizophrenic disorder. The meaning of differential diagnostics of obsessive-compulsive phenomena for adequate pharmacotherapy is discussed using the example of the individual case.