Increasing cesarean-section rates have focused attention on variations in the use of this procedure that appear to be independent of medical indication. We investigated the relation between the rate of primary cesarean section and socioeconomic status in a cohort of 245,854 singleton infants born to non-Hispanic white, black, Asian-American, and Mexican-American residents of Los Angeles County, California. On the basis of birth-certificate data for 1982 and 1983, a significant relation, independent of maternal age, parity, or birth weight, was found between the rates of primary cesarean section and socioeconomic status. Women who lived in census tracts with a median family income of more than $30,000 had a primary cesarean-section rate of 22.9 percent, as compared with 13.2 percent among women residing in areas with a median family income under $11,000. In women between the ages of 18 and 34, the incidence of reported complications of pregnancy or childbirth in the lowest-income group was 10.9 percent, as compared with 17.4 percent in the highest-income group (accounting for 42 percent of the difference in the rate of primary cesarean section between groups); the rate of primary cesarean section in the presence of complications in these two groups was 65.4 percent and 79.3 percent (accounting for 17 percent of the difference); and the primary rate in the absence of reported complications in these two groups was 6.4 percent and 10.5 percent (accounting for 41 percent of the difference). The rates of primary cesarean section were highest among non-Hispanic whites (20.6 percent), intermediate among Asian Americans (19.2 percent) and blacks (18.9 percent), and lowest among Mexican Americans (13.9 percent). Significant socioeconomic differences in these rates were observed in all four groups (P less than 0.01). We conclude that the rates of primary cesarean section vary directly with socioeconomic status and that this association cannot be accounted for by differences in maternal age, parity, birth weight, race, ethnic group, or complications of pregnancy or childbirth.