To evaluate the prevalence of dihydropyrimidinuria (DHPuria), we analyzed urine samples from 21,200 healthy Japanese infants, and found two cases of DHPuria without clinical symptoms. Based on this result, we estimated the prevalence to be approximately 1/10,000 births in Japan. In addition, we analyzed pyrimidine catabolism on a previously reported family with an adult DHPuria case. We newly identified the sister of the propositus as the second case of DHPuria in this family, because she excreted large amounts of dihydrouracil and dihydrothymine. The parents and the child of the propositus showed slight increases of dihydrouracil and dihydrothymine. This is the first family with 2 cases of DHPuria, indicating that DHPuria is an inherited condition. To determine the inheritance of DHPuria in this family and to examine the risk of 5-fluorouracil (5-FU) toxicity, a uracil loading test was performed on the parents. Urinary dihydrouracil concentrations in the parents after the loading were several times higher than those in normal control persons, the finding being consistent with DHPuria heterozygotes. This, along with data on the propositus, his sister, and his child, indicates that DHPuria is an autosomal recessive condition. In addition, DHPuria homozygotes may have a high risk of 5-FU toxicity, while the risk is relatively low in heterozygotes.