&NA;A randomized trial including 294 patients was performed to evaluate the prophylactic effect of singlevs. multiple doses of antibiotics in elective colorectal surgery. All patients received 1.5 g metronidazole and 3.0 g ampicillin peroperatively and were randomized to: 1) no further prophylactic antibiotic treatment, or 2) ampicillin 1 g×3 and metronidazole 0.5 g×3 given intravenously during the second and third postoperative days. Deep wound infection was seen in 9/149 (6 percent) receiving a single dose and in 8/145 (6 percent) receiving multiple doses. no differences were found in the two groups between frequencies of anastomotic dehiscences, intra‐abdominal abscesses, sepsis, and pulmonary infections. the two groups were similar according to distribution of sex, age, diagnosis, and type of surgery. A single peroperative dose of metronidazole and ampicillin is a simple and satisfactory antibiotic prophylaxis in elective colorectal surgery.