IN certain sphingolipidoses, the age at onset of the disease correlates with the amount of detectable activity of the disease-related enzymes. Brady 1 has previously shown an inverse correlation between the amount of residual enzyme activity and the rate of progression of clinical symptoms in both Gaucher's and Niemann-Pick disease. More recently a similar relation has been noted for the different forms of GM1 and GM2 gangliosidoses. 2, 3 The occurrence of both early and late onset variants of sulfatide lipidosis or metachromatic leukodystrophy (MLD) after periods of apparently normal development has been explained theoretically in a similar manner. 4 MLD is. © 1971, Massachusetts Medical Society. All rights reserved.