The main drawback with augmentation mammaplasty using implants is capsular contracture. The cause of this complication is still unknown. Silicone particles, hematoma, and bacterial contamination are some of the etiologic factors discussed. In this randomized, double-blind study on 76 breast-augmented women, 50 percent of the patients had preoperative prophylaxis with benzylpeni-cillin and dicloxacillin. Bacteria samples were taken intra-operatively. The number of negative cultures increased significantly with antibiotic prophylaxis. In four follow-ups during the first postoperative year, the rate of contractures was evaluated by subjective and objective methods. The results showed no statistically significant difference between the placebo and the antibiotic group with respect to the incidence of capsular contracture.