OBJECTIVE The purpose of this study was to determine whether 3 days of broad-spectrum antibiotic therapy, which is intended to prolong latency in patients with preterm premature rupture of membranes, is comparable to 7 days of therapy. METHODS Patients with preterm premature rupture of membranes at three separate study sites were asked to participate in this intent-to-treat, prospective, randomized trial. They were assigned randomly to either 3 or 7 days of ampicillin-sulbactam (3 g intravenously every 6 hours). The primary outcome of interest was the latency period from membrane rupture to delivery. RESULTS Forty-two individuals were enrolled in each group. No difference was noted in the latency interval between the two groups (3 days, 214 +/- 225 hours, vs 7 days, 229 +/- 218 hours). A significantly higher number of patients in the 3-day group completed therapy (80.1% vs 47.6%, P =.003). No other parameters were significantly different between the two groups. No adverse events or trends were noted in either group. CONCLUSIONS There appears to be no difference in the latency period between 3 and 7 days of ampicillin-sulbactam antibiotic therapy. More patients are needed to exclude a type II error.