Tumor development at the site of ureterointestinal anastomosis is a recognized complication in patients with continent urinary diversions. Aerobic cultures of rectal urine samples from 30 patients with urinary diversions (26 ureterosigmoidostomies, two colon conduits, one ileal conduit and a Gersuny bladder) showed a complex bacterial flora containing nitrate-reducing organisms (Escherichia coli, Proteus and Klebsiella spp.). In comparison to normal bladder urine samples from control volunteers (n = 20), rectal urine samples from ureterosigmoidostomy patients (n = 26) showed a significant decrease (P less than 0.0001) in urinary nitrate (0.93 +/- 0.39 versus 0.27 +/- 0.23 mmol/l), a significant increase (P less than 0.0001) in urinary nitrite (not detected versus 29.24 +/- 39.93 mumol/l) as well as a significant increase (P = 0.013) in urinary N-nitroso compound excretion (57.33 +/- 33.87 versus 93.96 +/- 65.76 nmol/l). Significant increases were also found for the urinary excretion of individual volatile and non-volatile N-nitroso compounds, clearly demonstrating a bacterially mediated in vivo formation of N-nitroso compounds in the 'colon' bladders of patients with urointestinal diversions that may be an important etiological risk factor for colon carcinogenesis in this patient group.