ABSTRACT— A retrospective study was performed to evaluate the effect of long-term treatment with clozapine in 96 schizophrenic or schizoaffective patients hospitalized at the Psychiatric Research Center in Uppsala during the period 1974-1986. All patients had previously been treated with different neuroleptics but with inadequate response or distressing side effects. The mean duration of the disorder at the start of clozapine treatment was 8 years and 9 months and the mean duration of the treatment 3 years and 11 months. Clozapine treatment was discontinued in 36 % of the patients, mainly due to lack of efficacy, poor compliance or temporary withdrawal of the drug from the market in 1975. In two patients the reason was leukopenia or agranulocytosis. In another 10 patients a transient decrement of WBC was seen, which was normalized during ongoing treatment. Four patients died when on clozapine during the follow-up period, but no causal relationship to the treatment could be established. Eighty-five per cent of the patients could be discharged from the hospital. Of the 62 patients who were still on clozapine after 2 years, 18% had full time and 21 % half-time employment. A global evaluation of the clinical efficacy revealed a significant improvement in 43 % and a moderate improvement in 38 % of the patients compared to previous neuroleptic treatments. Common but usually mild side effects were sedation, hypersalivation, weight gain, and obstipation. Four patients had grand mal seizures. No extrapyramidal side effects were observed during clozapine treatment. It is concluded that clozapine is an effective antipsychotic drug that offers particular advantages for many schizophrenic patients compared to classical neuroleptics and that the risk/benefit ratio should be re-evaluated in the light of recent long-term studies.