PURPOSE: To evaluate the role of colonic motility in the pathogenesis of anorectal symptoms and dysfunction after radiotherapy (RT) for carcinoma of the prostate. PATIENTS AND METHODS: Thirty-eight patients, median age 71 (range, 50–81) years with localized prostate carcinoma randomized to one of two radiation dose schedules underwent colonic transit scintigraphy and assessment of anorectal symptoms (questionnaire), anorectal function (manometry), and anal sphincteric morphology (endoanal ultrasound) before and at 1 month and 1 year after RT. RESULTS: Whole and distal colonic transit increased 1 month after RT, with faster distal colonic transit only persisting at 1 year. Frequency and urgency of defecation, fecal incontinence, and rectal bleeding increased 1 month after RT and persisted at 1 year. Basal anal pressures remained unchanged, but progressive reductions occurred in anal squeeze pressures and responses to increased intra-abdominal pressure. Rectal compliance decreased progressively in the patients, although no changes in anorectal sensory function ensued. Radiotherapy had no effect on the morphology of the internal and external anal sphincters. Distal colonic retention was weakly related to rectal compliance at 1 month, but both faster colonic transit and reduced rectal compliance were more frequent with increased fecal urgency. At 1 year, a weak inverse relationship existed between colonic half-clearance time and frequency of defecation, although both faster whole-colonic transit and reduced rectal compliance occurred more often with increased stool frequency. CONCLUSION: Colonic dysmotility contributes to anorectal dysfunction after RT for carcinoma of the prostate. This has implications for improving the management of anorectal radiation sequelae. ; Eric K. Yeoh, Dylan L. Bartholomeusz, Richard H. Holloway, Robert J. Fraser, Rochelle Botton, Addolorata Di Matteo, James W. Moore and Mark N. Shoeman