Twenty-three cases of microvillous atrophy were reviewed to determine clinical and morphological characteristics of the disease. Congenital and late-onset forms of presentation were clearly identified in which the late-onset cases appeared to have a better prognosis. Three different, and distinctive, appearances of the proximal small intestinal mucosa were found. Careful orientation of mucosal samples allowed a temporal sequence of events to be delineated in which the first morphological abnormality to be detected in the epithelium was the accumulation of "secretory granules"; microvillous inclusions were seen in older cells in the upper villous region. It is suggested that, in familial microvillous atrophy, diarrhoea and disorganisation of the brush border assembly occur as a consequence of a more fundamental defect that affects the intracellular traffic of certain cell components, as indicated by the accumulation of "secretory granules."