The usefulness of antibiotic prophylaxis in reducing the incidence of postoperative infections following colorectal surgery has been defined in many clinical studies. Since a single antibiotic agent covering all the potential pathogens (i.e. Gram-negative aerobes and anaerobes) is not currently available, a combination therapy is usually administered. The present study reports the preliminary results on comparative efficacy and safety of aztreonam and gentamicin (both combined with clindamycin) as short-term prophylaxis in colorectal surgery. Four hundred and fifty-four patients, out of 495 enrolled, were considered evaluable. Incidence of abdominal wound infections was significantly lower in the aztreonam-treated group (5.8% vs 12.5% - p less than or equal to .025). The incidence of perineal wound and abdominal infections, of fever, of need for postoperative antibiotics and the length of postoperative stay were lower in the monobactam group, even if none of these differences were statistically significant. Gram-negative aerobic microorganisms were more often isolated in infected patients from the gentamicin group. Side effects were minor and similar in the two groups. From these preliminary data, the aztreonam-clindamycin combination can be considered a major candidate to replace many of the presently available oral and parenteral regimens.