OBJECTIVE This study addressed the unique clinical properties attributed to the atypical antipsychotic clozapine, including its efficacy in patients with treatment-refractory psychosis and against negative symptoms, its lack of acute extrapyramidal side effects, and the longer time course of its therapeutic effects. METHODS The clinical responses of 84 schizophrenic inpatients (66 with treatment-refractory illness and 18 who were intolerant of antipsychotic treatment) were examined. After all previous antipsychotic medications had been withdrawn, the patients were treated with clozapine according to a standardized titration and dosage schedule. Patients who tolerated and responded to treatment were discharged and maintained on a regimen of clozapine for up to 52 weeks. Patients were evaluated for behavioral response and side effects after weeks 3, 6, 12, 26, 39, and 52 of treatment. RESULTS Fifty percent of the patients with treatment-refractory illness and 76% of the treatment-intolerant patients responded to clozapine in up to 52 weeks. The optimal period for a trial of clozapine appeared to be 12-24 weeks. Clozapine exhibited therapeutic effects on negative symptoms, but these were not clearly independent of its effects on positive symptoms and extrapyramidal side effects. Several variables, including early age at onset of illness and female gender, were found to be predictors of poor response to treatment. Predictors of good response included the presence of extrapyramidal side effects during previous treatment with classic neuroleptics and a diagnosis of paranoid schizophrenia. CONCLUSIONS These findings have important implications for the use of clozapine and our understanding of the pathophysiology of treatment-resistant schizophrenia.