We performed detailed studies of renal function in two of five related patients with normal serum creatinine levels to determine the mechanism of their chronic azotemia. Inulin and para-amino-hippurate clearances, maximum tubular transport of para-aminohippurate, and renal acidification were within normal limits. In addition, renal concentrating and diluting abilities of these patients were similar to those of four normal controls. Urea clearances of both patients during maximum water diuresis (27.6 and 40.8 ml per minute per 1.73 m2) and antidiuresis (5.3 and 4.0), however, were much lower than mean (+/- S.E.M.) values in the normal controls (70.4 +/- 3.7 and 30.0 +/- 3.42 ml per minute per 1.73 m2, respectively). Thus decreased urea excretion despite otherwise normal renal function was responsible for the chronic azotemia of these patients. The genetic defect in renal urea clearance appeared to be inherited as an autosomal dominant trait.