A 48-year-old man presented with giant folds of the gastric mucosa, hypoalbuminemia, and hypersecretion of acid. In vivo labeling of plasma proteins by 51CrCl3 disclosed an abnormal gastric protein clearance. Mucus secretion, evaluated by the output of dry matter, mucosubstance constituents (hexosamine and fucose), protein (by the method of Lowry), and sodium were five to eight times those in nine healthy volunteers, but were normalized by the histamine-H2-receptor antagonist cimetidine. The proportion of dry matter and protein was the same in the patient and in healthy subjects, indicating that the loss of plasma protein into the stomach probably occurred as part of an abnormally great secretion of mucus.