ABSTRACT: Since 1981, the cesarean birth rate of a joint practice has been consistently lower than that of physician-only practices at a private community hospital in Yolo County, California. This study sought to determine whether differences in perinatal outcomes were influenced by women's use of a joint versus a physician-only practice or were associated with parity, maternal age, or newborn birthweight. Data from the hospital's 1634 consecutive singleton births in 1990 were examined, using a prospective concurrent analytic cohort study design. Chi square statistics and stepwise logistic regressions were used for data analysis. The joint practice had a significantly lower rate of total cesarean births (9.3%) compared with the physician-only practices (17.7%); the frequencies of severe lacerations were 1.0 percent and 6.4 percent, respectively. No significant differences were found in parity, birthweight, or newborn outcomes in the two types of practice. Type of practice was the major determinant of cesarean birth (p > 0.0001). All variables studied, including type of practice, were significant determinants of primary cesarean birth. Parity and practice type were significant determinants of third- and fourth-degree lacerations (p > 0.0001). The type of practice from which women receive care is significantly associated with both method of birth and possibility of severe perineal trauma.