Abstract The effect of hygienic measures in two groups of hip operations was compared. Forty patients had total hip replacement operations (THR) representing elective surgery, and 38 patients had open osteosynthesis of pertrochanteric fractures (PFO) representing acute surgery. THR was performed on non-infected cases after careful skin disinfection whereas PFO was often performed after less effective patient preparation. No patient had antibiotic prophylaxis. The surgical team was dressed in gowns of conventional type. The operating theatre had a so-called zonal ventilation system. The number of airborne bacteria in the operating theatre during operation varied from 6 to 160 cfu/m 3. The mean values were significantly higher during THR than during PFO and the operation time was longer. The operation wound area contained significantly higher levels of airborne bacteria than other parts of the room. Cultures from adhesive drapes showed growth in 33 out of 40 THR and in 24 out of 38 PFO. In the latter group clostridia were found on the drapes more often than in the THR group. Swabs from the wounds taken at the end of the operation showed growth in five THR and in six PFO. The rate of superficial infection was 13 per cent in the THR group and 15·8 per cent in the PFO group. The deep infection rate was 5·4 and 0 per cent, respectively. Most of the infections seemed to be endogenous.