One hundred and thirty-two male patients undergoing upper abdominal surgery were investigated clinically, radiologically, and with ventilatory function tests before and after operation in a controlled trial in order to assess the effect of prophylactic Crystamycin, regular analgesia with morphine, and isoprenaline inhalations on the incidence of postoperative chest complications. Crystamycin, given in a total dose of 10 mega units of sodium penicillin G + 10 g. of streptomycin sulphate over a period of five days starting on the day before operation, reduced the incidence of serious chest complications by more than 50%; no benefit was observed from the use of regular analgesia with morphine or from isoprenaline inhalations. With the use of preoperative ventilatory function tests and a detailed history, an attempt was made to define a group of patients with an increased risk of developing a postoperative chest complication. In this series the older patients were found to have a significantly greater risk ; those with a preoperative history of wheeze, productive morning cough, and impaired ventilatory function had a slightly, but insignificantly, increased risk. No other characteristics of the patients examined were found to influence the risk. © 1968, British Medical Journal Publishing Group. All rights reserved.