The effect of five days of antibiotic prophylaxis with cefazolin injections (beginning just before surgery) on postoperative infections (beginning just before surgery) on postoperative infectious complications was evaluated in a double-blind, randomised, placebo-controlled trial in nine centres on 2137 patients undergoing hip replacement. Antibiotic prophylaxis reduced the number of hip infections significantly from 3.3% (placebo) to 0.9% (cefazolin). Positive peroperative blood samples and positive bacteriological examination of the drain were risk factors for hip infection but the prognostic value of obesity, diabetes, or previous hip surgery was not confirmed. Development of a urinary infection was not related to hip infection. Hip infections were less common in the four centres with hypersterile operating theatres, and the benefits of prophylactic antibiotics were restricted to patients having hip replacement operations in conventional theatres.