We studied 11,814 women admitted for labor and delivery to 84 free-standing birth centers in the United States and followed their course and that of their infants through delivery or transfer to a hospital and for at least four weeks thereafter. The women were at lower-than-average risk of a poor outcome of pregnancy, according to many but not all of the recognized demographic and behavioral risk factors. Among the women, 70.7 percent had only minor complications or none; 7.9 percent had serious emergency complications during labor and delivery or soon thereafter, such as thick meconium or severe shoulder dystocia. One woman in six (15.8 percent) was transferred to a hospital; 2.4 percent had emergency transfers. Twenty-nine percent of nulliparous women and only 7 percent of parous women were transferred, but the frequency of emergency transfers was the same. The rate of cesarean section was 4.4 percent. There were no maternal deaths. The overall intrapartum and neonatal mortality rate was 1.3 per 1000 births. The rates of infant mortality and low Apgar scores were similar to those reported in large studies of low-risk hospital births. We conclude that birth centers offer a safe and acceptable alternative to hospital confinement for selected pregnant women, particularly those who have previously had children, and that such care leads to relatively few cesarean sections.