For 1 year all patients in the Department of General Surgery who were transferred to the intensive care unit (n = 237) were prospectively studied as to the development of hyponatremia. Relevant hyponatremia developed in 6.3% of the patients. The lowest sodium concentration observed was 121 mmol/l, the mean duration was 1.2 days. In general, hyponatremia is related to hypovolemia and diuretics are often involved. In no patient were there any clinical consequences.