OBJECTIVE We determine the effect of urinary diversion with a Kock ileal reservoir on bile acid absorption and bowel habits. METHODS We asked 96 patients with a Kock ileal urinary reservoir to record bowel habits and abdominal symptoms for 1 week. Data on 75 patients were further analyzed. Bile acid absorption was determined in 29 healthy control subjects, in 17 before and 6 months after continent urinary diversion, and in 21, 2 to 14 years postoperatively. Bile acid absorption was considered pathological when retention of less than 10% of an oral capsule containing selenium-75 labeled tauroselcholic acid (SeHCAT) was noted after 1 week. RESULTS Mean number of defecations plus or minus standard deviation was 9.4 +/- 6.1 (75 cases). Of the patients 13% had 15 or more stools per week and 15% complained of always having loose stools. Mean value for the SeHCAT test was 32 +/- 19% preoperatively and 17 +/- 16% 6 months postoperatively (p = 0.0023). The corresponding value for healthy controls was 39 +/- 18%. Significant relationships were found between the results of the SeHCAT test postoperatively, and the number of stools per week and consistency of the feces. All patients with more than 10 defecations per week had a pathological SeHCAT test. CONCLUSIONS Most patients with an ileal urinary reservoir have fairly normal bowel habits. Bile acid absorption is significantly reduced postoperatively and approximately a third of the patients have a pathological SeHCAT test. Preoperative investigation of bowel habits is recommended and a SeHCAT test should be performed in patients with frequent, loose defecations. Other types of diversion should be offered when preoperative retention is below 10 to 20% especially in patients with impaired anal control.