An increased amount of N-acetylaspartic acid was found in urine and plasma of three patients, from two families, with the diagnosis of cerebral spongy degeneration (Canavan disease). Aspartoacylase was assayed in cultured skin fibroblasts from one patient of each family and a profound deficiency of this enzyme was found. Although the function of N-acetylaspartic acid is not understood, it is known to occur in high concentration in human brain. The finding of a defect in the metabolism of N-acetylaspartic acid causing progressive spongy degeneration of the brain may lead to a better understanding of the function of this amino acid derivative. The aspartoacylase assay affords a new tool for determining the diagnosis of Canavan disease. Since aspartoacylase activity was present in cultured amniotic cells and chorionic villi, it is likely that the assay for this enzyme can be used for the prenatal diagnosis of Canavan disease.