Background Low anterior resection syndrome (LARS) is a frequent problem after rectal resection. Transanal irrigation (TAI) has been suggested as an effective treatment in patients who have developed LARS. This prospective RCT was undertaken to evaluate the effect of TAI as a prophylactic treatment to prevent symptoms of LARS. Methods Patients who had undergone ultralow rectal resection were randomized to start TAI on a daily basis, or to serve as a control with supportive therapy only after ileostomy closure. All patients were seen after 1 week, 1 month and 3 months, and the maximum number of defaecation episodes per day and night documented during follow‐up. Wexner score, LARS score and Short Form 36 questionnaire responses were evaluated in both groups. Results Thirty‐seven patients could be evaluated according to protocol (TAI 18, control 19). The maximum number of stool episodes per day and per night was significantly lower among patients who underwent TAI at 1 month (median 3 versus 7 episodes/day in TAI versus control group, P = 0·003; 0 versus 3 episodes/night, P = 0·001) and 3 months (3 versus 5 episodes per day, P = 0·006; 0 versus 1 episodes/night, P = 0·002). LARS scores were significantly better in the TAI group after 1 month (median 16 versus 32 in control group; P = 0·044) and 3 months (9 versus 31; P = 0·001). A significantly better result in terms of Wexner score was seen in the TAI group after 3 months (median 2 versus 6 in controls; P = 0·046). Conclusion Prophylactic TAI led to a significantly better functional outcome compared with supportive therapy for up to 3 months. Registration number: DRKS00011752 ( http://apps.who.int/trialsearch/).

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