Five time-related peripartum events were prospectively studied in 101 high-risk, internally monitored parturients. These included the duration of labor, the duration of ruptured membranes, the number of vaginal examinations, the length of time from first vaginal examination to delivery, and the duration of internal monitoring. Seventy patients were delivered vaginally and 31 by cesarean section. Several of the time-related peripartum events were found to be individually related to postpartum morbidity. However, combined evaluation by stepwise discriminant analysis revealed that the duration of labor alone was the primary determinant of postpartum morbidity. These findings imply that the observation of a relationship between postpartum morbidity and any of several peripartum time-related factors in isolation may be mediated by its relationship in time to labor duration. This suggests, in turn, that intensive labor monitoring techniques in high-risk patients would not be expected to increase postpartum morbidity above that expected from increases in labor duration alone. The results further suggest that the duration of labor may be the only time-related peripartum event useful in evaluating the risk for postpartum infection in monitored patients in labor who undergo cesarean section.