A three-year-old boy has coarse facial features, upper respiratory congestion, profound mental retardation, hepatosplenomegaly, increased height and head circumference, cataracts, a gibbus deformity, radiographic changes of dysostosis multiplex, and vacuolized peripheral lymphocytes. These findings are the most commonly reported clinical features in the previously described patients with mannosidosis. Our patient has a severe deficiency, and his parents have intermediate levels, of the acidic component of alpha-mannosidase in their cultured fibroblasts.