Extract: Two sisters, one a neonate and the other 3 years of age, were found to suffer from pyroglutamic aciduria (5-oxoprolinuria). They had a chronic metabolic acidosis and required therapy with bicarbonate. Unlike their parents, both excreted large quantites of L-pyroglutamate in the urine. No pyroglutamate could be demonstrated in a urine sample obtained from the pregnant mother at 30 weeks of gestation. Pyroglutamate was, however, detected in amniotic fluid and cord blood plasma as well as in urine from the neonate collected during the first few hours after birth. Metabolic acidosis developed during the 1st day of life and bicarbonate therapy was started. The patient also showed a moderate hyperbilirubinemia but no further complications were encountered in the neonatal period. The 3-year-old girl had a daily excretion of pyroglutamate in urine corresponding to 195 mmol/1.73 m, i.e., identical with that found at 14 months of age. The excretion showed no correlation with feeding or physical activity. The concentrations of pyroglutamate in plasma and cerebrospinal fluid were 2.2 mM and 1.3 mM, respectively. She had no evidence of a disturbed renal transport of α-amino acids. Her psychologic and somatic development was normal, and she had no signs of neurologic damage. Both patients presented evidence of increased hemolysis and their levels of glutathione in erythrocytes were markedly decreased, which indicated a defect in glutathione metabolism. Speculation: In our patients with pyroglutamic aciduria, the levels of pyroglutamate-degrading enzyme (5-oxoprolinase) in leukocytes and cultured fibroblasts were found to be normal and turnover studies revealed a high capacity for the breakdown of pyroglutamate. These observations make a defect in pyroglutamate degradation somewhat unlikely. Instead, we believe that the patients have an increased production of pyroglutamate. The erythrocyte glutathione level was markedly reduced and it is suggested that pyroglutamate is formed in excessive amounts because of an enzymatic defect in the glutathione synthesis.