The perinatal mortality rate in California decreased rapidly in the 1970s; neonatal mortality fell about twice as fast as fetal mortality. Decreases in birth-weight-specific mortality accounted for 81 per cent of the decline in the perinatal rate, with only 19 per cent due to improvements in birth weight. No improvement was observed in the birth-weight distribution for blacks. The decrease in mortality rates was significantly faster for cesarean deliveries than for vaginal births. By 1977, all birth-weight-specific fetal mortality rates for cesarean sections were equal to those for vaginal deliveries or lower. For infants weighing less than 2000 g, perinatal mortality rates were also significantly lower for infants born by cesarean section than for infants delivered vaginally. These results suggest that much of the recent decrease in perinatal mortality rates can be attributed to the advent of neonatal intensive care and the increased rate of cesarean section.