To determine the role of antibiotic prophylaxis for stab wounds of the chest requiring intercostal tube drainage, a double-blind study was conducted comparing cefazolin, given to 57 patients at 500 mg every 8 h for 24 h, with placebo, given to 56. Differences were detected in the rate of thoracotomy for sepsis (antibiotic nil versus placebo 9 per cent, P < 0.05) and in the frequency of sputa positive for pathogens (12 versus 34 per cent respectively, P < 0.05), but the incidence of pyrexia, raised white cell count, positive cultures from pleural drainage or intercostal catheter tips and volume of chest drainage was similar. The mean hospital stay and costs consequent on morbidity were greater in patients receiving placebo, supporting the conclusion that antibiotic prophylaxis is indicated.