We describe a patient with glutaryl-coenzyme A dehydrogenase deficiency, demonstrated by a residual enzyme activity of only 1% in cultured fibroblasts. Although the clinical presentation was typical of glutaric aciduria type I, the urine concentrations of glutaric, glutaconic, and 3-hydroxyglutaric acids remained normal, even during episodes of clinical decompensation. An increased free glutarate level was demonstrated only in cerebrospinal fluid.