Objective: To identify risk factors associated with brachial plexus injury in a large population. Methods: A computerized data set containing records from hospital discharge summaries of mothers and infants and birth certificates was examined. The deliveries took place in more than 300 civilian acute care hospitals in California between January 1, 1994, and December 31, 1995. Cases of brachial plexus injury were evaluated for additional diagnoses and procedures of pregnancy, such as mode of delivery, gestational diabetes, and shoulder dystocia. Those complications were stratified by birth weight and analyzed, using bivariate and multivariate techniques to identify specific risk factors. Results: Among 1,094,298 women who delivered during the 2 years, 1611 (0.15%) had diagnoses of brachial plexus injury. The frequency of diagnosis increased with the addition of gestational diabetes (odds ratio [OR] 1.9, 95% confidence interval [CI] 1.7, 2.1), forceps delivery (OR 3.4, 95% CI 2.7, 4.3), vacuum extraction (OR 2.7, 95% CI 2.4, 3.1), and shoulder dystocia (OR 76.1, 95% CI 69, 84). In cases of brachial plexus injury, the frequency of shoulder dystocia increased from 22%, when birth weight ranged between 2.5 and 3.5 kg, to 74%, when birth weight exceeded 4.5 kg. The frequency of diagnosis of other malpresentation (nonbreech) (OR 73.6, 95% CI 66, 83) was increased for all birth weight categories. Severe (OR 13.6, 95% CI 8.3, 22.5) and mild (OR 6.3, 95% CI 3.9, 10.1) birth asphyxia were increased. Prematurity (OR 0.8, 95% CI 0.67, 0.98) and fetal growth restriction (OR 0.1, 95% CI 0.03, 0.40) were protective against brachial plexus injury. Conclusion: In macrosomic newborns, shoulder dystocia was associated with brachial plexus injury, but in low- and normal-weight infants, “other malpresentation” was diagnosed more frequently than shoulder dystocia. Our study findings suggest that brachial plexus injury has causes in addition to shoulder dystocia and might result from an abnormality during the antepartum or intrapartum period.