Objective To determine the safety of the conservative management of ureteric calculi of >4 mm in diameter, using mercapto-acetyltriglycine (MAG3) radioisotope renography to monitor renal parenchymal function. Patients and methods Patients with symptomatic unilateral ureteric calculi were recruited prospectively. After con®rming the position of the stone using contrast urography, a MAG3 radioisotope renogram was taken within 48 h of admission and again 1 month after the patients became stone-free. The indications for intervention were ipsilateral loss of function (o5% loss), infection, pain or any combination of these factors. The recovery of function was determined by follow-up renography. Results In all, 54 patients were recruited; 18 were initially allocated to conservative treatment although four later required intervention for pain. The remainder required early intervention for pain (eight), diminished function only (15) or diminished function with infection (13). Of the 54 patients, 28% had silent' loss of renal function at presentation. No calculi of >7 mm diameter passed without intervention. The mode of initial management was determined according to individual clinical need. The upper tracts of all patients were relieved of obstruction and all patients were rendered stone-free. Intervention for reduced function only (at ¡7 days) gave a signi®-cantly better (P=0.03) recovery of function than those undergoing later intervention. The combination of reduced function and infection were unrelated to outcome after intervention. Conclusions The conservative management of stones of 5±7 mm diameter is safe, providing that radioisotope renography is used to identify those renal units requiring intervention. This approach allows timely intervention to maximize the recovery of renal function.