Glutaric aciduria type I (GA-I) is an inborn error in the degradation of lysine, hydroxylysine, and tryptophan due to a deficiency of glutaryl-CoA dehydrogenase. Glutaric, 3-OH-glutaric, and glutaconic acids are excreted in the urine, particularly during intercurrent illness. The enzyme may be assayed in leukocytes, cultured fibroblasts and chorionic villi. Twelve new cases, 9 months-16 years of age, are reported, comprising all known cases of GA-I in Sweden and Norway. Ten had a severe dystonic-dyskinetic disorder, one had a mild hyperkinetic disorder, and one was asymptomatic. Two children died in a state of hyperthermia. Carnitine deficiency and malnutrition developed in patients with severe dystonia and dysphagia, which necessitated substitution and gastrostomy. A slowly progressive dyskinetic disorder developed in spite of adequate early dietary treatment in one subject. Macrocephaly was found in three. Computed tomography and magnetic resonance investigations in 10 showed deep bitemporal spaces in 7. Neuropsychological testing of 8 of 12 subjects demonstrated receptive language function to be superior to expressive language and motor function. Cognitive functions were obviously less affected than motor functions. A review of 57 pooled cases showed that a severe dystonic syndrome developed in 77%, a mild extrapyramidal syndrome in 10%, and 12% were asymptomatic. This disorder may pass undetected in the cerebral palsy and mentally retarded child and adult populations. Repeated urine examinations of organic acids in the urine and enzyme assay may be necessary to confirm GA-I.