Multidisciplinary studies were conducted on the brain and other tissues of patients who died with the antemortem diagnosis of mucopolysaccharidosis (MPS) of one of the following types; type V, Scheie disease (MPS-V); type I, Hurler disease (MPS-I): and type II, Hunter disease (MPS-II). The principal new finding in the brain of the patient with MPS-V is the presence of lesions in the periadventitial mesenchymal tissue of the white matter, similar to those of MPS-I, while the nerve cells in MPS-V are histologically normal, in contradistinction to MPS-I, in which the neuronal abnormality is severe. Electron microscopical studies of the brain in MPS-I demonstrated numerous complex membranous inclusions in the neurons, whereas the neurons in MPS-V contained only a small number of lipofuscin-like inclusions and typical lipofuscin granules. There was a threefold increase of glycosaminoglycans (GAG) in the brain of MPS-I, but only a slight increase in the MPS-V; GAG in the liver and spleen of all patients was noticeably increased. alpha-L-iduronidase activity was not detectable in the brain and liver of patients with MPS-I and MPS-V, thus suggesting a similar enzymatic defect.