We have assessed the independent predictors of duration of active labor in nulliparous women at term. Using a cohort of 1067 nulliparae in spontaneous labor at > 37.0 weeks with singleton fetuses in vertex presentation, multivariate analysis was used to identify independent predictors of duration of active labor. Duration of active labor was 4.1 +/- 2.4 hours. Stepwise linear regression selected 10 independent predictors of duration of active labor: gestational age at delivery ( P < 0.001), race ( P = 0.014), obstetric risk factors ( P = 0.022), amniotomy ( P < 0.001), fundal height ( P = 0.005), cervical dilation on admission ( P < 0.001), frequency of contractions ( P < 0.001), station of presenting part ( P < 0.001), oxytocin ( P < 0.001), and epidural use ( P < 0.001). A prediction formula incorporating the 10 predictors accounted for 51% of the total variance of the observed duration of active labor. Ten variables are independent predictors of duration of active labor; when incorporated in a prediction formula they account for > 50% of the variability of duration of labor in nulliparous women.