Peroperative tinidazole and placebo treatment were compared with respect to incidence of postoperative infections in a double blind investigation on patients admitted for elective bowel surgery. Patients in the tinidazole group had a significantly reduced postoperative infection rate, compared with the placebo group (7.4% and 46%, respectively). Complications in general were fewer in the tinidazole group and costs for antibiotic treatment decreased. All tinidazole-treated patients had clinically effective blood concentrations of the drug at the time of the operation. In contrast with the placebo-treated subjects, the individuals who developed postoperative wound infections from the tinidazole group had no growth of anaerobic bacteria and no antibody response to Bacteroides sp. The use of tinidazole may therefore be recommended for the prevention of postoperative anaerobic infections in patients undergoing bowel surgery.