Analysis of the neurologic symptomatology in 22 patients with Niemann-Pick disease type C revealed 3 phenotypes: (1) an early-onset, rapidly progressive form associated with severe hepatic dysfunction and psychomotor delay during infancy and later with supranuclear vertical gaze paresis, ataxia, marked spasticity, and dementia; (2) a delayed-onset, slowly progressive form heralded by the appearance, usually in early childhood, of mild intellectual impairment, supranuclear vertical gaze paresis, and ataxia, and later associated with dementia and, variably, seizures and extrapyramidal deficits; (3) a late-onset slowly progressive form distinguished from the 2nd pattern by later age of onset (adolescence or adulthood) and a much slower rate of progression. The existence of the 1st and 2nd phenotypes within the same sibship suggests that they are variant expressions of the same clinicopathologic disorder. Niemann-Pick disease type C should be considered not only in infants and children who present with organomegaly and a progressive neurodegenerative course, but also in adolescents and adults who have insidiously progressive neurologic dysfunction and only slight organomegaly. Associated with the disease is a marked deficiency in the ability of cultured fibroblasts to esterify exogenously supplied cholesterol. Assay of this deficiency is particularly useful for confirming the diagnosis in patients with atypical presentation.