Biopsies of rectal mucosa and autopsy samples of duodenal, jejunal and ileum obtained from a 4-month old infantwere studied by light and electron microscopy. The patient had congenital, watery diarrhea with fecal volumes of up to 300 ml/kg/day. In the jejunum villi were shortened, and the epithelium did not have a visible brush border, the apical cvytoplasm was filled with PAS-positive material, a similarly staining material was also present in the supranuclear area, probably in the Golgi apparatus. The architecture of the rectal nucosa was also normal; however, the luminal surface of the epithelial cells was convex. With the electrom microscope the brush border of the surface epithelium is altered, forming cytoplasmic bridges with irregular microvilli on both surfaces. Most microvilli tend to be located in the proximity of the intercellular junctions. The apical third of the cytoplasm is filled with curved membrane segments whose thickness is similar to that of the surface membrane or of the few microvilli present (97 nm); these membrane segments are sometimes interconnected by their ends and may conform cavities that open to the colonic lumen. Thin filaments that resemble the glycolalix fill thel concavity of the membranes. Microvillus inclusion was observed in one cell. The other cell organelles were normal. This could be a different form of microvillus inclusion disease, in which membranes accumulate without forming either microvilli or a brush border. Diarrhea is probably due to loss of polarity of these cells. The assembly of the brush border is a process that may become interrupted at any stage, generating different morphologic appearances.