A highly circadian incidence of premature rupture of the fetal membranes was found in the Collaborative Perinatal Project data base. Among term deliveries without membranitis, the peak incidence of premature rupture of the membranes occurred at 3:00 AM, and the amplitude of a fitted sinusoid was 45%. Rupture hour was markedly influenced by latent interval and by membranitis. In the absence of membranitis, in term cases with a latent interval of less than three hours, rupture hour peaked at 2:00 AM, with an amplitude of 64%. Those with a latent interval of three to 35 hours peaked at 4:15 AM, with an amplitude of 37%. Those with 36 or more hours' latent interval showed little circadian variation. Preterm premature rupture of the membranes was also segregated into subgroups by latent interval. Preterm premature rupture of the membranes with a three- to 35-hour latent interval peaked at 4:00 AM, with an amplitude of 31%, similar to the corresponding term group. A circadian factor thus appears to cause most cases of premature rupture of the membranes in term and preterm births when membranitis is absent. The observed stratification by latent interval indicates that the mechanisms of premature rupture of the membranes may differ between the subgroups. With membranitis, the early-morning peak in premature rupture of the membranes was not statistically demonstrable in any of the term or preterm latent interval subgroups. This indicates that there may be unique rupture mechanisms associated with membranitis. The data in this study should assist efforts to classify premature rupture of the membranes and study its mechanisms.