We tested the effectiveness of a single dose of prophylactic antibiotic (gentamicin) in elective cholecystectomy in a double-blind, controlled randomized study. All patients recognized preoperatively as being at risk were excluded. The treatment group comprised of 102 patients received a single dose of gentamicin and the 74 patients in the control group received a placebo. Of the patients who received gentamicin, wound infection developed in 4.9 percent versus 13.5 percent in the control group. Among 45 patients who had positive bile cultures, the wound infection rate for those in the treatment group was 14 percent versus 44 percent for those in the control group. Of 17 patients who underwent unexpected exploration of the choledochus, none of those in the treatment group had development of wound infection. The rate of wound infection in the control group was 50 percent. As 30 percent of the patients undergoing elective cholecystectomy were found to have risk factors for the development of wound infection which could not be identified pre-operatively, we recommend single dose prophylaxis for all patients undergoing cholecystectomy.