A mentally retarded girl with epileptic seizures is described. Urinary organic acid screening revealed a massive excretion of glyceric acid, a normally barely detectable metabolite. Hyperglycinemia was not observed. Capillary gas chromatography of the O-acetylated (-)- menthyl ester of urinary glyceric acid showed the substance to have the D-configuration. The urinary D-glycerate excretion remained unaltered after an oral load with 200 mg/kg L-serine, but oral loading with fructose (1 g/kg) or dihydroxyacetone (1 g/kg) caused a sharp increase of the Dglycerate excretion. Treatment with a diet moderately restricted in fructose led to some clinical improvement as judged by subjective criteria. The metabolic lesion is thought to be located at some step of the fructose catabolic pathway, possibly at the level of hepatic triokinase deficiency. (Pediatr Res 21: 502-506,1987)