In order to determine whether glomerular hyperfiltration in diabetes is related to renal prostaglandin production we have studied the urinary excretion of PGE2, 6-keto-PGF1 alpha, and TXB2 in two sex, age and duration of diabetes matched groups of 9 and 10 Type 1 diabetic patients with either normal (mean 121, range 105-129 ml min-1 1.73 m-2) or supranormal glomerular filtration rate (154, 135-206 ml min-1 1.73 m-2). A group of 15 matched healthy volunteers served as control subjects. Urine was collected overnight for an uninterrupted period of at least 6 h. All studies in the patients were performed during insulin-induced sustained euglycaemia to prevent the confounding effect of variable degrees of blood glucose control on urinary prostaglandin excretion. Blood pressure was normal in all subjects. Urinary excretion of 6-keto-PGF1 alpha was significantly higher in the patients with glomerular hyperfiltration (median 17.1, range 4.5-33.6 ng h-1) than in those without (8.8, 1.5-13.8 ng h-1; p less than 0.05) or in normal control subjects (9.6, 5.2-15.5 ng h-1; p less than 0.05). No significant differences were found in the excretion rates of PGE2 and TXB2 between the three groups. Under conditions of controlled plasma glucose and insulin concentrations the urinary excretion of 6-keto-PGF1 alpha, the stable breakdown product of PGI2, a compound of endothelial, possibly glomerular, origin was elevated only in the diabetic patients with glomerular hyperfiltration.