Clozapine, as the model agent for the atypical antipsychotic drugs, is currently recommended as effective regarding negative symptoms of schizophrenia and treatment-resistant schizophrenic patients. This study focuses on the clozapine-induced side-effects in 100 hospitalized schizophrenic patients (negative and therapy-resistant forms), followed-up for a four year period. Clinically relevant side-effects occurred in 73% of all patients. Tachycardia (67%), the increase of liver enzymes (36%), hypotension (29%) and sedation (27%) were most frequent. Tachycardia, hypotension and sedation disappeared during the initial phase of treatment (i.e. 4-6 weeks), as tolerance developed with continuation of therapy. The increase of liver enzymes appeared to be dose related, since the reduction of daily clozapine dose led to the normalization of transaminases values. The other side-effects (constipation, nausea and vertigo) were rare and transient. Leucopenia was not registered in any patient during the follow-up period. Therefore, clozapine is efficient and, with some precautions concerning hepatotoxicity, is safe for in- and outpatient long-term treatment in appropriately selected patients.