PURPOSE: The aim of this study was to determine the outcome and identify predictors of success of biofeedback therapy for patients with internal rectal intussusception. METHODS: Twenty-one patients (median age. 54 years, 15 female, 6 male) diagnosed with internal rectal intussusception by using cinedefecography were evaluated by standardized questionnaire, before, immediately after treatment, and at follow-up. Clinical bowel symptoms and anorectal physiological studies were also analyzed. RESULTS: Follow up (median: 15, range 2~24 months) results were evaluated by an independent observer in 20 patients. At post-biofeedback, 20 (95%) patients felt improvement in symptoms, including 7 (33%) with complete symptom relief. At follow-up, 17 (85%) patients felt improvement in symptoms, including 7 (35%) with complete symptom relief. There was a significant reduction in difficult defecation (from 90 to 29, 10 percent, from pre-biofeedback to post-biofeedback, and at follow up respectively; P<0.001), sensation of incomplete defecation (from 90 to 24, 35 percent; P<0.001), enema use (from 29 to 0, 0 percent; P<0.01), and anal pain (from 19 to 0, 0 percent; P<0.05). Digitation (from 19 to 0 percent, from pre- biofeedback to at follow up; P<0.05) and laxative use (from 29 to 5 percent; P<0.05) were also decreased. Normal spontaneous bowel movement was increased from 52 percent, at pre-biofeedback to 86 percent, at post- biofeedback (P<0.05), 95 percent at follow up (P<0.005). Low bowel frequency, at pre-biofeedback (P<0.01), and hard stool (P<0.05) predicted poor outcome. Long puborectalis length during push (P<0.05) and dynamic descent (P<0.05) on defecography also predicted poor outcome. Low electrical activities of puborectalis and anal sphincter muscle during rest (P<0.001), squeeze (P<0.01) and push (P<0.005) on pre-biofeedback electromyography were related to poor outcome. No patient developed full rectal prolapse during follow up. CONCLUSIONS: Biofeedback is an effective option and should be considered as the first line therapy especially for patients that don't have low bowel frequency, long puborectalis during push, and dynamic descent before treatment.