Three brothers with mannosidosis were studied, and their clinical and biochemical manifestations are compared with those of 41 cases in the literature. All three boys have psychomotor and growth retardation, characteristic facies, recurrent respiratory infections, sensorineural deafness, craniosynostosis, protuberant abdomens, and thin limbs. Roentgenographic findings of mild dysostosis multiplex, thick calvaria, abnormally contoured vertebrae, coarse trabeculi and thin cortices are consistent with those of reported cases. The lymphocytes of peripheral blood and bone marrow are vacuolated. Alpha-mannosidase deficiency in leukocytes and cultured skin fibroblasts and glycoproteinuria have been documented. The biochemistry of this glycoproteinosis and the pitfalls in diagnosis, such as improper assay conditions of pH and substrate concentration, are discussed. Extrapolation of in vitro and animal model studies suggest that trace metal therapy may be more effective than attempts at enzyme replacement to treat this hereditary storage disease.