Brush border membrane proteins from a child with congenital microvillus atrophy were analysed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and compared with brush border membrane proteins from normal and disease controls. A predominant band of MW 200K was present in all preparations with the exception of the membrane preparation from the child with congenital microvillus atrophy. Although the band was present in the latter case, it was grossly diminished. The diminished 200K MW band may result in the abnormality of brush border structure. This could account for the involution of the microvilli, the distinguishing feature of congenital microvillus atrophy, and could be the underlying defect in this disease.