Dihydropyrimidine dehydrogenase (EC 1.3.1.2.) deficiency has recently been described in two separate reports of four patients, all of whom excreted large quantities of thymine and uracil in the urine. Bakkeren et al. (1984) described a girl who developed grand-mal epilepsy at 3 years of age, and had microcephaly, but normal intelligence. Berger et al. (1984) reported on three patients, a boy with transient petit-mal and behavioural abnormalities developing at 18 months, a mentally retarded girl aged 14 years with solitary behaviour and petit-mal, and a boy with microcephaly, mental retardation and probable growth retardation, whose symptoms first appeared at 9 months. A further report of excretion of thymine and uracil was made by Berglund et al. (1979) in a 2-year-old with a medulloblastoma. However, severe deficiency of dihydropyrimidine dehydrogenase could not be demonstrated in this patient, in studies of cultured fibroblasts.