ALTHOUGH HEREDITARY orotic aciduria and refractory megaloblastic anemia associated with retardation of growth and development was described by Huguley et al1 as a genetic defect in pyrimidine metabolism in 1959, it was not until 1965 that additional cases were identified.2-4 The purpose of this paper is to describe one of these cases in greater detail and to call further attention to the successful use of uridine, a pyrimidine nucleoside, in replacement therapy. Report of a Case The patient was born on May 8, 1962, the product of a 35-week gestation, and his birth weight was 1.8 kg (4 lb). He was first taken to a physician at 7 months of age because of weakness. At ten months of age, he was referred to the University of Texas Medical Branch for evaluation of anemia, resistance to iron therapy, and developmental retardation. The parents had little knowledge of their family background,