Severe hyponatremia has been described after elective surgery with subsequent permanent brain damage. Other authors, however, have noted that morbidity and mortality rates of severe hyponatremia have been greatly overestimated. We retrospectively examined 19 patients (8 male, 11 female) who developed severe hyponatremia (100 to 124 mmol/liter) after transsphenoidal surgery for pituitary adenomas. Eight patients had hormonally inactive adenomas, 5 ACTH-secreting adenomas, 2 GH-secreting adenomas and 4 prolactin-secreting adenomas. The mean age of the patients was 47.5 years, with a range from 16 to 71 years. The mean preoperative serum sodium level was 137.8 mmol/liter. The timing of hyponatremia showed two different patterns. Five patients developed early postoperative hyponatremia (mean 114.0 mmol/liter +/- 4.85) and 14 patients showed the lowest mean serum level one week after surgery (118.1 mmol/liter +/- 6.86). Patients with early hyponatremia had fewer and less severe symptoms than patients with delayed hyponatremia. None of the patients developed seizures or a demyelination syndrome. Despite severe degree of hyponatremia for most of our patients treatment with water restriction and oral sodium supplementation was sufficient.