A comparison was made between the excretion urogram and the radio-isotope renogram in 35 patients with uretero-sigmoid anastomosis. A good correlation was obtained between the 2 tests in the presence of radiographic evidence of (a) hydronephrosis, (b) poor renal function and (c) absent function. Renographic evidence of mild impairment of secretory function and of delayed excretion was associated with a significant number of normal excretion urograms and in a few instances there was later progression to severe renal impairment or hydronephrosis. Hydro-ureter associated with a normal renogram carried a good functional prognosis. Renography is recommended in the long-term supervision of patients with uretero-sigmoid anastomosis.