The late-onset form of GM2 gangliosidosis (Tay-Sachs disease) is an autosomal-recessive disorder with progressive neurologic disease, mainly characterized by motor neuron and spinocerebellar dysfunction. The majority of patients are of Ashkenazi Jewish origin. 31Phosphorus magnetic resonance spectroscopy of the brain was performed to study the metabolic changes of a 16-year-old patient with late-onset Tay-Sachs disease who had a heterozygous Gly269-->Ser mutation in the hexosaminidase A encoding gene in compound heterozygosity with another, yet unidentified mutation. Severe changes in phosphorus metabolism with a decreased amount of phosphodiesters and membrane-bound phosphates were demonstrated, suggesting an activation of phosphodiesterases by accumulating gangliosides. The clinical findings were well related to the changes in spectroscopically determined metabolites.