15 consecutive persons aged under 50 with an overweight exceeding 75% were examined clinically, biochemically and with liver biopsy after gastric bypass (7 patients) or gastroplasty (8 patients). After 1 year the occurrence of steatosis had fallen from 73 to 40% which, together with a marked decrease in individual gradings of fatty changes, represented a significant regression of the steatosis. Likewise, discrete inflammatory and granulomatous changes largely disappeared. In no case was fibrosis present. Serum alkaline phosphatases preoperatively increased above normal range in 20% but their mean level was significantly reduced after 12 months of weight loss. Other liver function tests remained normal and stable. In contrast to published experience with jejunoileal bypass operations liver steatosis associated with morbid obesity seems to be morphologically and biochemically reversed together with the weight reduction obtained by gastroplasty or gastric bypass.