The divided renal function of 10 patients with unilateral renal obstruction was measured with the renal perfusion/excretion determination renogram, a computerized radioisotope function study, and with endogenous creatinine clearances. Functional impairment of the obstructed kidney when estimated by the effective renal plasma flow determination of the renal perfusion/excretion determination renogram correlated closely with that measured by creatinine clearance. The clinical applications of the renal perfusion/excretion determination renogram in unilateral renal obstruction include the preoperative evaluation of individual renal function and serial estimates of individual renal function in chronic obstructive processes.