Two hundred and sixteen psychiatric patients (183 men and 33 women) hospitalized in Sct. Hans Hospital were treated with clozapine between 1971-1983. All had been treated previously with one or more neuroleptic(s) and had either failed to respond adequately, or their response was limited by side effects. Eighty-five patients were treated exclusively with clozapine, while the remaining 131 received additional medication, mainly other neuroleptic drugs. The mean clozapine dosage was 317 mg/day (range 50-1200), and the mean duration of treatment was 23/4 years (range 1/12-12). The tolerability to clozapine was determined by an evaluation of haematological changes, pronounced side effects and mortality. One patient treated with clozapine (8 months) and nitrofurantoin (8 days) developed a reversible granulocytopenia. One patient (treated with a combination of drugs) had clinically insignificant depression of the leucocytes and three of segmented granulocytes. Seven had a reduction in thrombocytes. Two patients developed cardiac insufficiency, and four epileptic seizures. None of the patients treated exclusively with clozapine developed neurological side effects. A global estimation of therapeutic effect revealed that clozapine alone or in combination with other neuroleptic drugs was significantly better than previous antipsychotic therapy, although 47-63% of the patients showed no change. It is concluded that clozapine is a potent antipsychotic drug offering particular advantages in the treatment of schizophrenic patients with a pronounced symptomatology and tendency towards developing extrapyramidal side effects. Caution is advised in patients with cardiac insufficiency and epilepsy. There appears to be a slight risk of granulocytopenia, and therefore the present monitoring of WBC should continue in order to prevent this reaction and to obtain more complete information regarding risk of granulocytopenia.