Urodynamic studies of the ileal conduit were performed in 36 patients with normal upper urinary tracts (controls) and in 32 who had progressive upper tract dilatation within 6 years preceding the study. In controls the conduit emptied mainly by means of low pressure, to-and-fro activity. In contrast, high frequency, high amplitude peristaltic activity was found commonly in patients with upper tract dilatation, such activity being consistent with obstruction of the distal conduit. Of the 32 patients with upper tract dilatation 17 underwent reoperation on the conduit as well as postoperative urodynamic studies. Among these 17 patients a decrease was observed in basal pressure in the conduit postoperatively compared to preoperatively (5 +/- 4 and 12 +/- 6 cm. water, p less than 0.001). A decrease also was found postoperatively in the frequency (4 +/- 6 and 36 +/- 28 per hour, p less than 0.001) and amplitude (34 +/- 47 and 61 +/- 28 cm. water, p less than 0.001) of peristaltic activity. Radiological improvement in the appearance of the upper urinary tracts was found in 9 patients (53 per cent) postoperatively. These findings support the hypothesis that upper tract dilatation in certain patients with an ileal conduit may be caused by high pressure activity in the presence of free ureteral reflux. Such abnormalities, which are probably the result of obstruction of the distal conduit, may be detected by means of urodynamic evaluation and may be corrected by revision of the conduit.