Defective jejunal sodium/proton exchange causes severe, congenital secretory diarrhea. We report a boy who presented typically in utero, but in whom diarrhea resolved during the first year of life. Pregnancy was complicated by polyhydramnios, and an ultrasound at 31 weeks showed a distended fetal small intestine. The abdomen was grossly distended at birth, and profuse secretory diarrhea began immediately. He subsequently thrived on breast milk and electrolyte supplements. Studies of jejunal brush border sodium/proton exchange at 6 months showed a partial defect. Nonequilibrium rectal dialysis showed rectal sodium and potassium transport to be intact. Diarrhea lessened after 9 months, and the patient subsequently required occasional laxatives. These observations suggest that there is a spectrum of congenital abnormality in this exchanger, and that in children with incomplete defects normal colonic sodium salvage can subsequently mask net small intestinal secretion.