Congenital alkalosis or chloride-losing diarrhea is a rare abnormality of intestinal electrolyte transport. We describe the clinical course and metabolic features of a 31-year-old man, the only known adult with this disease. The illness is characterized by sustained watery diarrhea, an acid stool, metabolic alkalosis, and a unique stool chloride concentration of 140 meq/litre. Although the stool volume and cation composition varies with intake of NaCl, the stool chloride concentration and pH remain unchanged under all conditions of NaCl balance. The renal response is appropriate to the metabolic state; other ions in plasma, urine, and stool are essentially normal. Additional unexplained clinical features include glucose intolerance, gout, proteinuria, and mild renal functional impairment. The features of this disease can be explained by a defect in the ileal transport of chloride, which results in a high stool chloride concentration, selective chloride depletion, and, secondarily, metabolic alkalosis.