Nine patients with a progressive neurologic disorder that was characterized by mental deterioration, supranuclear vertical gaze paresis, and foam cells or sea-blue histiocytes in the bone marrow are described and compared with patients who were previously described as having " neurovisceral storage disease with vertical supranuclear ophthalmoplegia" and "dystonic lipidosis." The clinical manifestations of our patients and those described by others and the pathologic findings and profiles of lipid analysis reported by others are similar to those in patients with Niemann-Pick disease, type C. Sphingomyelinase activities in leukocytes and skin fibroblasts were normal in our patients and in more than half of the reported cases; these findings are also compatible with those in patients with Niemann-Pick disease, type C. Until the biochemical and genetic abnormalities of Niemann-Pick disease, type C are clearly defined, it is justifiable to classify the disorder under discussion as a subgroup of Niemann-Pick disease, type C because it seems to be a heterogeneous group. From the clinical point of view, the diagnosis is difficult to establish in the absence of abnormalities in the bone marrow in patients who are older than 20 years; repeat examinations of the bone marrow are necessary in such patients. Clinicians should be aware of this disorder not only in patients in the first and second decades of life, when this disorder usually becomes symptomatic, but also in patients in the fourth and fifth decades.