To evaluate the incidence and causes of hyponatremia in intensive care unit (ICU) patients, retrospective and prospective studies were done. Hyponatremia was defined as a serum sodium concentration equal to or less than 134 mmol/l (134 mEq/l). Prospectively, 29.6% of patients displayed hyponatremia. Relevant data were obtained in twelve patients. Two patients did not have serum hypoosmolality. In the ten patients with serum hypoosmolality, urine osmolality was not maximally dilute and urine sodium concentration was greater than 30 mmol/l (30 mEq/l) suggesting inappropriate antidiuretic hormone secretion (SIADH). However, three patients exhibited suppressed ADH levels despite absence of maximal urinary dilution. The data suggest that hyponatremia is common in ICU patients and that renal diluting defects are frequent. Therefore, hypotonic fluid should be administered cautiously.