In a family with hydroxyprolinemia the proposita was a 31-year-old woman whose disorder was characterized by the accumulation of large amounts of free hydroxyproline in the plasma, with overflow into the urine. Biochemical studies suggested that the accumulation of free hydroxyproline in the proposita, as in the only previously reported case, was due to deficiency of hydroxyproline oxidase, the enzyme that normally converts free hydroxyproline to Δ¹-pyrroline-3-hydroxy-5-carboxylic acid. Unlike the previous patient, the proposita was not mentally retarded, and did not seem to have any clinical abnormality that could have been due to the metabolic defect. Studies on 20 relatives indicated that an apparently healthy 27-year-old brother of the proposita likewise had hydroxyprolinemia, and his two-year-old daughter had slightly elevated endogenous hydroxyproline values. The pedigree was consistent with autosomal recessive inheritance.