&NA;A prospective, double‐blind, randomized study was undertaken to compare perioperative parenteral metronidazole and erythromycin. One neomycin, and cefazolinhundred fifty‐five patients were randomized into two groups by the pharmacy department. The resulting difference between the overall septic complication rate in patients receiving erythromycin, neomycin, and cefazolin (10.9 percent) and the rate in patients receiving metronidazole alone (31.9 percent) was significant. This indicates that an antibiotic to cover aerobic bacteria should be added to the regimen when metronidazole is used.