A two-year-old boy with a malignant tumor of the brain (medulloblastoma) excreted large amounts of thymine and uracil in his urine. The excretion was related to progress and regress of the disease, and reached a maximum of 3.0 mol of thymine per mole of creatinine and 2.6 mol of uracil per mole of creatinine. The excretion by 20 apparently normal children was less than 0.01 mol/mol of creatinine for each of the two pyrimidines. Three children with brain tumors, two with leukemias, and one with neuroblastoma were also studied; two of them had a moderate increase in urinary pyrimidine excretion, but only up to 0.07 mol/mol of creatinine. The activity of dihydrouracil dehydrogenase (NADP+) (EC 1.3.1.2) in cultured fibroblasts from the patient was somewhat lower than in control fibroblasts. The tumor was considered to be the likely cause of the increased excretion of pyrimidines, but an impaired degradation of pyrimidines in the liver could not be ruled out.