Caesarean sections are performed with an ever increasing frequency, and their morbidity rate due to infection varies between 35 and 40 per cent. The effectiveness of antibiotic prophylaxis has been demonstrated in caesarean sections with a high risk of infection, but few studies have been devoted to caesarean sections without this high risk. The purpose of our study was to evaluate the effectiveness of antibiotic therapy in this second type of caesarean section since its postoperative infection rate is not negligible (about 25 per cent). We therefore set up a two-centre randomized trial comparing two groups of 133 women without any particular risk of infection. At the moment of umbilical cord clamping, one group received cefotetan 1 g intravenously, while the other group received an intravenous injection of a placebo solution. The postoperative infection rate was 12.5 per cent in the treatment group and 26.9 per cent in the control group (P less than 0.05). The relative risk in the placebo-treated women was 2.15 (95 per cent confidence limits, 1.41 to 3.28). Antibiotic prophylaxis prevented 53.5 per cent of postoperative infections (95 per cent confidence limits, 29 to 69.5 per cent). Moreover, in the treatment group infections were less severe, resulting in a significant decrease in hospital stay and a lower overall cost. We conclude that antibiotic prophylaxis with cefotetan is effective in caesarean sections without a high risk of infection, as it significantly reduces the postoperative morbidity due to infection.