A prospective study of 2983 operations in general and orthopaedic surgery during 3 years performed in four operating theatres in a modern operating suite was carried out in order to evaluate the importance of airborne infection. Weekly nose-and-throat samples were taken from the surgical staff and pre-operative samples were taken from the nose, throat, skin and perineum of the patients. The air contamination was followed by using settle plates, which showed low mean counts of total bacteria of between 9 and 15 c.f.u./m(2)/min, with mean counts of Staph. aureus of between 0.03 and 0.06 c.f.u./m(2)/min. No correlation was found between the total number of bacteria and the incidence of post-operative infections or between the amount of Staph. aureus in the air and post-operative Staph. aureus infections. It was concluded that further increases in ventilation could, at best, only marginally affect the incidence of post-operative infection.The post-operative wound-infection rate was 9.0%. In various types of surgery, the infection rates varied from 5.3% in clean operations to 47.6% in dirty surgery. About one third of the infections were classified as moderate or severe.Adverse patient factors, such as immunodeficiency, steroid treatment, intensive care, etc., increased the rate to 15.0%; in ;normal' patients it was 3.8%.Among the bacteria isolated, gram-negative bacilli (31% of wounds), often together with other bacteria, and Staph. aureus (28%) predominated, but in 25% no specimens were taken.Of 76 post-operative Staph. aureus infections, 32 were caused by the patients' own strains, and of the remaining 44 infections, 22, or 8% of all infections, could be traced to strains present in the air and/or the respiratory tracts of staff during the operation.The length of pre-operative hospital stay had no influence on the carrier rate of Staph. aureus in patients. The incidence of post-operative wound infection was significantly higher in patients carrying Staph. aureus and was even higher if these bacteria were found on the skin.Patients with wound infections stayed, on an average, 15 days longer than patients without infections. In serious infections the increase in duration of stay was > 20 days. Although infections were commoner in older patients, the average additional hospital stay of infected patients did not increase with age. If the post-operative infections studied in a concurrent retrospective study are taken into account more than 12 000 bed-days were due to post-surgical wound infections in the period studied or, in other words, some 12 beds (corresponding to 5.5% of the total) were always occupied by infected patients.