Radioisotope renograms with orthoiodohippurate-I131 were obtained for 41 children who had known renal or renovascular disorders. (Two patients had hypertension but no evidence of renal disease.) Values determined for various components of the renogram curves were considered abnormal when they exceeded two standard deviations from the mean values for normal children of similar body surface area or when time B (from injection to the point of maximal radioactivity) exceeded 6.5 minutes or differed from the contralateral kidney by more than 1 minute. The renogram was clearly abnormal in all 17 patients who had unilateral or predominantly unilateral renal disorders. Abnormalities associated with bilateral renal disorders varied with the disease and its severity. The clinical severity of impairment of renal function could be estimated from the renogram. The isotope renogram proved to be a simple, reproducible technique for determination of function of the separate kidneys.