We have studied 5 infants with persistent severe diarrhea from birth and marked abnormalities of absorption associated with failure to thrive leading to death in 4 infants. Three had siblings who died and a sibling of a 4th is ill at present, all with a similar illness; 2 were the products of consanguinous marriages. Exhaustive investigation failed to identify a recognized disease entity in any patient. Steatorrhea, sugar malabsorption, dehydration, and acidosis were severe in all patients, whatever the diet fed. Total parenteral nutrition was used, but excessive stool water and electrolyte losses persisted even when nothing was fed by mouth. There was no evidence of a hematological or consistent immunological defect in any infant and no abnormalities of intestinal hormones were noted. In the duodenal mucosa of all infants we saw similar abnormalities characterized by villus atrophy, crypt hypoplasia without an increase in mitoses or inflammatory cell infiltrate in the lamina propria and in villus enterocytes absence of a brush border, increase in lysosome-like inclusions, and autophagocytosis. In 3 infants studied by marker perfusion of the proximal jejunum we found abnormal glucose absorption and a blunted response of Na+ absorption to actively transported nonelectrolytes; in 2 there was net secretion of Na+ and H2O in the basal state. Our patients evidently suffered from a congenital enteropathy which caused profound defects in their capacity to assimilate nutrients. The similar structural lesion seen in the small intestinal epithelium of all of our cases undoubtedly contributed to their compromised intestinal function, but the pathogenesis of this disorder, if indeed it is a single disease, remains obscure.