The occurrence of acute dystonia was prospectively monitored in 39 schizophrenic patients (18 male and 21 female) treated with 9 to 27 mg/day of nemonapride, a selective dopamine antagonist, and the relationship of acute dystonia with characteristics of patients and plasma concentrations of the drug and prolactin was investigated.Twenty (51.3%) of 39 patients had dystonic reactions, the onsets of which occurred within 3 days after the initiation of treatment in 90% of dystonic patients. The incidence of acute dystonia was significantly higher in male than in female patients (77.8% vs. 28.6%, p < 0.05). Younger male patients (<or=to 30 years) especially had an extremely high incidence of this side effect (91.7%). A positive correlation between prolactin response after 1 week of treatment and dystonia rating scores was found in male (Spearman rank correlation: rs= 0.606, p < 0.01) but not in female patients (rs= -0.378, p = not significant). These results suggest that young male patients have the highest risk of neuroleptic-induced dystonia. Prolactin response after 1 week of treatment as an index of dopamine blockade may reflect vulnerability to the development of acute dystonia at least in male patients treated with nemonapride. (J Clin Psychopharmacol 1999;19:45-50)