Background & Aims: Biofeedback is reported to be as effective for slow transit constipation as for pelvic floor dyssynergia and no more effective than education. We aimed to test the hypothesis that biofeedback benefits only patients with pelvic floor dyssynergia, describe the physiologic mechanism of treatment, and identify predictors of success. Methods: Fifty- two patients (49 women; average age, 35 years), all with delayed whole gut transit, included 34 with pelvic floor dyssynergia, 12 with slow transit only, and 6 who met only 1 of 2 criteria for pelvic floor dyssynergia. All received 5 weekly biofeedback sessions directed at increasing rectal pressure and relaxing pelvic floor muscles during straining plus practice defecating a balloon. Patients were retested by questionnaire; symptom diary; balloon defecation; transit study at 1, 6, 12, and 24 months; and anorectal manometry at 1 and 6 months. Results: At 6 months, greater improvements were seen in pelvic floor dyssynergia compared with slow transit only; 71% versus 8% reported satisfaction (P = .001), and 76% versus 8% reported < 3 bowel movements per week (P < .001). Improvements were maintained at 24 months of follow- up. Biofeedback eliminated dyssynergia in 91% and enabled 85% to defecate the balloon. Satisfaction was correlated with improved ability to defecate the balloon (ρ = .73; P < .001), reductions in dyssynergia (ρ = .69; P < .001), and increased rectal pressure during straining (ρ = .36; P < .01). Success was predicted by pelvic floor dyssynergia, milder constipation, and less frequent abdominal pain at baseline. Conclusions: Biofeedback is an effective treatment for pelvic floor dyssynergia but not slow transit constipation. Background & Aims: Biofeedback is reported to be effective for slow transit constipation as for pelvic floor dyssynergia and no more effective than education. We aimed to test the hypothesis that biofeedback benefits only patients with pelvic floor dyssynergia, describe the physiologic mechanism of treatment, and with predictors of success. Methods: Fifty-two patients (49 women; average age, 35 years), all with delayed whole gut transit, included 34 with pelvic floor dyssynergia, 12 with slow transit only, and 6 who met only 1 of 2 criteria for pelvic floor dyssynergia. All received 5 weekly biofeedback sessions directed at increasing rectal pressure and relaxing pelvic floor muscles during straining plus practice defecating a balloon. Patients were retested by questionnaire; symptom diary; balloon defecation; transit study at 1, 6, 12, and 24 months; and anorectal manometry at 1 and 6 months. Results: At 6 months, greater improvements were seen in pelvic floor dyssynergia comp ared with slow transit only; 71% versus 8% reported satisfaction (P = .001), and 76% versus 8% reported <3 bowel movements per week (P <.001). Improvements were maintained at 24 months of follow- up . Biofeedback eliminated dyssynergia in 91% and enabled 85% to defecate the balloon. Satisfaction was correlated with improved ability to defecate the balloon (ρ = .73; P <.001), reductions in dyssynergia (ρ = .69; P <. 001), and increased rectal pressure during straining (p = .36; P <.01). Success was predicted by pelvic floor dyssynergia, milder constipation, and less frequent abdominal pain at baseline. Conclusions: Biofeedback is an effective treatment for pelvic floor dyssynergia but not slow transit constipation.