Hereditary orotic aciduria was found in a 7-year-old girl who had anemia, leukopenia, a megaloblastic bone marrow, crystalluria, and normal growth and intelligence. Uridine produced a rapid hematologic improvement, similar to that seen in the previously reported cases. The 24-hour excretion of urinary orotic acid ranged from 290 mg while the patient was on a dosage of 150 mg/kg/day of uridine, to 650 mg while she was on a dosage of 20 mg/ kg/day. The urinary orotic acid/creatinine ratio varied inversely with the uridine dose. Enzyme studies on hemolysates and cultured fibroblasts from the patient revealed grossly reduced levels of orotidine-5′-monophosphate pyrophosphorylase and decarboxylase. The red blood cell levels of both of these enzymes in the one parent studied were compatible with heterozygosity.