A randomized controlled trial has been performed to assess the value of plastic wound drapes in the prevention of surgical wound infection. One hundred and forty-four patients undergoing abdominal surgery were allocated to one of three groups; a control group (A) in which standard cloth towels were applied to the abdominal wound, group B in which an adhesive plastic drape was added and group C in which a plastic ring protector was inserted into the wound. There were 26 cases (18 per cent) of wound infection. The wound infection rate in the plastic drape groups did not differ significantly from that in the control group. Bacteriological culture from a wound swab taken at operation was positive in 32 per cent of the cases. The presence of a plastic wound drape did not influence the positive culture rate. In 68 per cent of wounds which became infected the operative swab was positive as compared with 24 per cent in those not developing a wound infection. Thirteen of the 33 patients (39 per cent) with a positive wound swab subsequently developed a wound infection. In 10 of these 13 cases of infection the operative swab permitted an accurate prediction of the organism responsible for the subsequent infection.