The effect of risperidone on polydipsia-hyponatremia was evaluated in six hospitalized schizophrenic patients. The normalized diurnal weight gain (NDWG), urine-specific gravity (USG), urine and plasma osmolarity, and serum sodium were monitored during 9 months of risperidone treatment. The dose of risperidone (mean +/- SD=8.0 +/- 1.0, range=6-9 mg/day) was determined as approximately half of the haloperidol-equivalent dose of previous neuroleptics. Before risperidone treatment, the mean (+/- SD) BPRS score was 23.5 +/- 7.1; no significant improvement was observed after risperidone (22.0 +/- 7.5). The subjects showed relatively high serum prolactin before risperidone treatment (mean +/- SD=16.5 +/- 9.7 ng/mL), that was not significantly decreased by risperidone (14.2 +/- 7.9 ng/mL). The monthly means (+/- SD) of NDWG and USG before risperidone were 5.5 +/- 1.5 (%) and 1.002 +/- 0.001, respectively. These and other indices did not significantly improve throughout the study period. Although the sample size is relatively small, our preliminary data showed that risperidone might not be effective on polydipsia-hyponatremia of schizophrenic patients.