The authors report on a Portuguese family with 3 adult brothers affected with GM2-gangliosidosis (B1 variant) in a sibship of 4, and more specifically on one of these brothers with neurological onset at the age of 17. Psychosis, lower motoneuron involvement and dysarthria were predominant in two of the cases; the third had a cerebellar symptomatology. Hexosaminidase A activity, studied in leukocytes, was profoundly deficient when measured using the specific sulfated substrate, but nearly normal using a conventional assay (non-sulfated substrate). These results established the diagnosis of the unusual enzymological form of GM2-gangliosidosis known as the B1 variant, which had so far not been associated with an adult phenotype. Molecular studies are in progress to study genotype/phenotype correlations in this family in comparison with known mutations in the B1 variant and in adult GM2-gangliosidosis. This report also emphasizes that a metabolic etiology, leading to genetic counselling, should be considered in some familial degenerative neurological disorders.