Two siblings with GM1-gangliosidosis and a patient with juvenile cerebral lipidosis were studied by clinical, histochemical, and chemical methods. GM1-gangliosidosis was of a late infantile variety without visceromegaly or skeletal abnormality but with the presence of PAS positive histiocytes in the liver and spleen. The degree of neuronal involvement was much greater in this condition than in the late juvenile lipidosis. In the GM1-gangliosidosis, the asialo derivative was increased, the GM1-ganglioside was sevenfold of normal and β-galactosidase activity in leukocytes was less than 5% of normal; in the parent examined this activity was in a heterozygous range. In the juvenile lipidosis the values of cholesterol and phospholipids in the cerebral cortex were subnormal and the ganglioside was normal; in the white matter the total gangliosides were increased but their pattern was normal.