The relationship between urine specific gravity (USG) and the incidence of polydipsia/hyponatremia in hospitalized schizophrenic patients was examined. On the basis of five independent USG values, we identified 49% (35 out of 72) of male and 22% (8 out of 37) of female patients with hyposthenuria (mean USG < or = 1.008). Review of 2 years of records of routine laboratory examinations identified 12 males with hyponatremia, 11 males with borderline hyponatremia, and 49 males with normal values; among female patients, only one patient with hyponatremia and three patients with borderline values were identified. A significantly lower mean USG (1.003 +/- 0.001) for the male patients with hyponatremia compared with the male patients with normal serum sodium data (1.011 +/- 0.005) was observed. All of the male patients with hyponatremia, as well as 18 of the 49 male patients with normal serum sodium values, exhibited hyposthenuria. The USG values of all the patients with hyponatremia were consistently quite low (< or = 1.005), whereas 14 of the 18 normonatremia/hyposthenuria patients had normal USG values (i.e. > or = 1.009) in at least one of five determinations; the mean USG values for most (16 of 18) of the normonatremia/hyposthenuria patients ranged from 1.006 to 1.008. Thus, many hospitalized schizophrenic patients exhibit hyposthenuria of varying degrees, but consistently low USG values are most suggestive of the risk of polydipsia/hyponatremia.