Renal function was evaluated in 44 patients with adult polycystic renal disease at different stages of the affection. In 38 patients single or serial determinations of the glomerular filtration rate and the effective renal plasma flow were performed. In 21 of these patients and in an additional group of six subjects, serial determinations of serum creatinine were obtained. The data show that renal function does not decrease at a constant rate between birth and endstage renal failure. It remains well preserved for many years but decreases rapidly at a later stage. The relationship between indices of renal function (glomerular filtration rate, PAH-clearance and/or reciprocal serum creatinine) and the age of the patients might be described adequately by assuming that the impairment in renal function is proportional to the growth of the cysts and that the radius of the cysts increase at a slow but constant rate. Possible contributory factors are secondary infection and hypertension.