OBJECTIVES: To characterize the prevalence and pathophysiology of anorectal dysfunction up to 2 yr following radiation therapy (RT) for localized carcinoma of the prostate. METHODS: Thirty-eight patients, median age 68 (range 60-82) yr with localized prostate carcinoma randomly assigned to one of two radiation dose schedules, underwent evaluation of the following variables of anorectal function before RT, as well as 4-6 wk and 1 and 2 yr after its completion: (1) symptoms, (2) anorectal motility, (3) anorectal sensory function, and (4) anal sphincteric morphology. RESULTS: There was a persistent increase in anorectal symptoms after RT. At 2 yr, bowel frequency, urgency, and fecal incontinence were increased in 50%, 47%, and 26% of patients, respectively. After RT, there were progressive reductions of (1) basal anal pressures, (2) anal pressures in response to squeeze and increased intra-abdominal pressure, (3) rectal compliance, and (4) rectal volumes associated with sensory perception and the desire to defecate. The thickness of the external anal sphincter increased with time after RT. No difference was observed between the patients in the two radiation dose schedules. CONCLUSIONS: Anorectal dysfunction following RT for prostate carcinoma is an underestimated cause of morbidity, which progresses with time. The prevalence and pathophysiology of anorectal dysfunction is similar after treatment with two commonly used radiation dose schedules.