Carbamazepine-induced hyponatremia has been reported in 21.7% of 61 patients with mental retardation who received the medication for a variety of reasons. We studied 40 patients with mental retardation receiving carbamazepine to determine the prevalence of hyponatremia. Overall, hyponatremia was found in only 5.0% of these patients. Correlations with sodium level and carbamazepine dose, serum drug level, and concomitant neuroleptic and anticonvulsant polytherapy were also examined. Treatment with carbamazepine resulted in a statistically, but not clinically, significant decrease in serum sodium levels in patients receiving anticonvulsant polytherapy. Decreases in serum sodium were not related to carbamazepine dose or blood levels. Only one patient with underlying schizophrenia and psychogenic polydipsia demonstrated clinically significant hyponatremia during carbamazepine therapy.