The present study was undertaken to determine the minimal effective antibiotic dosage in caesarean section prophylaxis. The study was conducted at the Academisch Ziekenhuis der Vrije Universiteit in Amsterdam (Amsterdam Free University Hospital) to compare the efficacy of one dose of cefoxitin (2 g) with three administrations of 2, 1 and 1 g respectively. In this prospective and double-blind study, 66 patients were given one dose and 72 patients received three doses. In terms of febrile morbidity, endometritis, wound infection, urinary tract infection and need for postoperative antibiotic therapy, the three-dose group showed fewer postoperative infections: as to wound infections (p less than 0.05) and therapeutic antibiotic use (p less than 0.025) these differences were statistically significant. The numbers of days of hospitalization after the caesarean section also showed differences: 10.6 +/- 2.6 versus 9.8 +/- 1.5 days in the one- and three-dose groups respectively (p less than 0.05). It is concluded that, contrary to several reports in the literature, prophylaxis consisting of three administrations of cefoxitin is to be preferred. No significant allergic or adverse reactions were observed in our patients.