Five patients with childhood dystonia associated with reduced CSF biopterin, responsive to levodopa, and characterized by diurnal and exertional variation are reported from the Developmental and Metabolic Neurology Branch, National Institute of Neurological and Communicative Disorders and Stroke, Bethesda, MD. Of 4 familial cases of dystonia with biopterin deficiency limited to the CNS and of unknown etiology, 2 brothers were more severely affected than 2 sisters, and the dystonia was complicated by hyperreflexia and extensor plantar responses indicative of associated pyramidal tract involvement. The fifth patient had a systemic deficiency of biopterin with hyperphenylalanemia and atypical phenylketonuria, and his dystonic rigidity was purely extrapyramidal and without spasticity. Patients were least symptomatic in the early morning or after a nap and became progressively weak, rigid and dystonic during the day with complete immobility in the afternoon or evening, and often unable to speak or to swallow. Improvement following Sinemet 10/100 began within 36 hours and benefit has been sustained during treatment for 18 months in 4 patients and 6 years in one, without apparent development of drug tolerance. [1]