Tay-Sachs disease (GM2 gangliosidosis I) is an autosomal recessive lysosomal-storage disorder confined to the central nervous system, resulting from deficiency of hexosaminidase A. The case presented is of a twelve-month-old girl brought to the hospital because of mental-motor deterioration and convulsions. She was the child of first cousins and had a history of the deaths of two siblings with the same manifestations. Generalized hypotonia, macrocephaly, hyperacusis and a retinal cherry red spot appearance were present. There was no organomegaly. The diagnosis of Tay-Sachs disease was made by means of absence of serum hexosaminidase A activity.