Polydipsia occurs frequently in chronic schizophrenic patients, some of whom develop intermittent hyponatremia. Most therapeutic efforts have tried to control the hyponatremia. Four schizophrenic patients, followed for more than one year, showed improvement on clozapine. Case 1 was an outpatient without history of hyponatremia who improved from polydipsia and psychosis. The last three were inpatients with polydipsia, intermittent hyponatremia, and psychosis who showed minimal improvement of psychosis but significant decrease in polydipsia and water intoxication. Case 2 relapsed to polydipsia when clozapine was discontinued on two occasions. Case 3 demonstrated polyuria during 39% of days before clozapine and in 0% of days after two weeks of clozapine. In case 4, most baseline sodium levels were abnormal, but all became normal after clozapine. A time-series analysis for intervention effects showed a significant effect of clozapine (p=. 017). The limited information provided by these case reports suggest the need for controlled studies of the clozapine effect on polydipsic patients.