One hundred and seventeen patients undergoing operations on the head and neck were investigated for the causes of infection. There was a close relationship between the pre-operative culture and the risk of infection. There was a greater risk of infection (10.7%) when the pharynx was opened than when it was not (0.0%). The operations most liable to infection are laryngopharyngectomy and those in which skin flaps are used. The isolation of pathogenic bacteria in the operative exudate did not in general predispose to the development of infection, which did not occur when these cultures were negative. Positive cultures obtained from the drain on 3 consecutive days were a significant indication that infection will probably occur.