Twenty-four chronic schizophrenic outpatients with a mean age of 37.21 years +/- 9.96 SD were treated with risperidone (RSP) at the dosage of 2-9 mg/die (mean 4.46 mg/die +/- 1.30 SD, mean 0.06 mg/kg +/- 0.01 SD) for a year. Clinical evaluation was assessed with the Brief Psychiatric Rating Scale (BPRS), Positive and Negative Symptoms Scale (PANSS), Extrapyramidal Side Effects Rating Scale (EPSE) and a checklist for Anticholinergic Side Effects (ACS) at T0, then after 1 (T1), 2 (T2), 3 (T3), 6 (T6), 9 (T9) and 12 (T12) months. RSP and 9-hydroxy-risperidone (9OH-RSP) plasma levels were determined at T12 by the HPLC method. BPRS and PANSS mean values showed a significant improvement during the study. No correlation between RSP dosage (mg/kg) and RSP, 9OH-RSP plasma levels or active moiety resulted. A positive correlation between age and active moiety was observed. A positive correlation between RSP and 9OH-RSP plasma levels was observed. A curvilinear relationship between active moiety and PANSS improvement (%) was observed. Patients with the higher PANSS amelioration showed RSP + 9OH-RSP plasma levels ranging from 15 to 30 ng/mL. RSP seems to be quite an effective drug. It seems, however, difficult to devise appropriate dose schedules and plasma level determination seems to be necessary in some cases.