There is known to be a high risk of malignancy at uretero-colic anastomoses and it has been postulated that bacteria catalyse both the local formation and the activation of carcinogens such as N-nitrosamines. If this theory is correct, then the risk of malignancy should be greatest when the prevalence of mixed urinary tract infection is greatest. Bacterial culture was performed on samples of urine obtained from 56 patients with three forms of urinary tract diversion: ureterosigmoidostomy, isolated ileal and colon conduits. As expected, the rectal urine of all patients with a ureterosigmoidostomy had a rich bacterial flora. However, 11 of the 15 patients with ileal conduits (73%) had significant mixed growths of bacteria in the loop urine, whereas only six out of 16 colon loop urines (37.5%) were infected, all being monobacterial infections. The implications of these bacteriological findings for carcinogenesis associated with urinary diversion are discussed.