A prospective, multicentre, randomized trial was carried out in 19 hospitals in order to compare the efficacy of amoxycillin-clavulanic acid with cefotetan as antibiotic prophylaxis in patients undergoing elective colorectal surgery. Since the main purpose of the study was to demonstrate equivalence between the two regimens, the protocol planned the inclusion of 200 patients. Eligible patients were randomly assigned to receive either amoxycillin-clavulanic acid (2.2 g) or cefotetan (2 g) in a single infusion on the induction of anaesthesia. Failure of prophylaxis was defined as occurrence of infection of intestinal origin, either minor (wound cellulitis) or major (abscess, peritonitis, septicaemia) within the 30-day postoperative period. Among 221 randomized patients, 208 (105 amoxycillin-clavulanic acid, 103 cefotetan) aged 66 +/- 12 years (mean +/- SD) were evaluated while 13 were withdrawn. Colorectal cancer was the indication for surgery in 73% of cases. Eleven (10 +/- 6%, 95% confidence interval) and 13 (13 +/- 7%) failures were observed in the amoxycillin-clavulanic acid and cefotetan groups (P = 0.63 chi-square test) respectively. Most infections occurred before the 10th postoperative day (8% failures at this time, estimated by the Kaplan-Meier method). The results of the trial demonstrate that amoxycillin-clavulanic acid and cefotetan have similar efficacy when used for prophylaxis of infection after elective colorectal surgery.