OBJECTIVE AND DESIGN: To study the potential effect of ranitidine on postoperative infectious complications following emergency colorectal surgery. A randomized, placebo-controlled, double-blind trial was carried out in three university clinics and two county hospitals in Denmark. PATIENTS AND TREATMENT: One hundred and ninety-four consecutive patients undergoing acute colorectal surgery for perforated and/or obstructed large bowel were randomized in a double-blind fashion to receive ranitidine 100 mg i.v. twice a day commencing at induction of anesthesia and continued for five days (group I) or i.v. placebo (group II). All patients were given 1.5 g metronidazole plus 3.0 g cefuroxime at the time of surgery. Patients with perforation of the colon or rectum were given metronidazole and cefuroxime for further 3 days. All patients were assessed daily until discharge from the hospital. Thirty patients were withdrawn from the study (for reasons such as other diagnosis, refused to continue, medication not given as prescribed). MAIN OUTCOME MEASURES: Patients were observed for signs of infectious complications; such as wound infection, intra-abdominal abscess, septicemia, and pneumonia. RESULTS: Both groups were similar with respect to age, sex, weight, duration of surgery, blood transfusions, and site of the procedure, as well as the histologic nature of the underlying disease process. However, the Mannheim Peritonitis Index (MPI) was significantly higher in group I compared with group II (p < 0.05). Wound infection, intraabdominal abscess, septicemia, and pneumonia were 12.9%, 5.2%, 3.8% and 14%, respectively in group I. In group II, the infectious complications were 16.1%, 6.8%, 6.9% and 22%, respectively. Twelve patients (13.8%) in the placebo group developed more than one complication compared with 5 patients (6.5%) in the ranitidine group. CONCLUSION: Ranitidine may have a beneficial effect on postoperative infectious complications in patients following acute colorectal surgery.