AbstractA 9‐year‐old boy showed a progressive generalized dystonia, with onset at the age of 4 years, combined with mental deterioration and behavioral disturbances. The values of β‐hexosaminidase activities studied in plasma, leukocytes, and fibroblasts obtained using two different substrates (MUG‐NAc and MUG‐NAc‐6‐S) were significantly reduced but higher than in Tay–Sachs disease and similar to those found in the juvenile chronic form of GM2 gangliosidosis. With anticholinergic therapy, for 1.5 years, the dystonic symptoms did not progress and the boy can still care for himself and attend school. The description of another case of the disease. clinically expressed as dystonia, corroborates the existence of dystonic phenotype of GM2 gangliosidosis.