Ethnic differences in preterm (less than 37 weeks) and very preterm (less than 33 weeks) delivery were evaluated in a prospective cohort of 28,330 women. Blacks had the highest rate of preterm and very preterm delivery, followed by Mexican-Americans, Asians, and Whites. Adjustment for maternal age, education, marital status, employment, parity, number of previous spontaneous or induced abortions, smoking and drinking during pregnancy, infant sex, and gestational age at initiation of prenatal care resulted in the following odds ratios for preterm delivery: 1.79 (1.55-2.08) for Blacks, 1.40 (1.19-1.63) for Mexican-Americans, 1.40 (1.16-1.69) for Asians, and 1.00 for Whites. The corresponding odds ratios for very preterm delivery were 2.35 (1.72-3.22) for Blacks, 1.31 (0.88-1.94) for Mexican-Americans, 1.10 (0.67-1.83) for Asians, and 1.00 for Whites. Exclusion of cases of premature rupture of membranes, placenta previa, and abruptio placenta did not explain the large ethnic differences. Although Whites and Mexican-Americans had similar birthweight distributions, Mexican-Americans had an increased risk for preterm delivery. Fifty-five per cent of low birthweight babies in Kaiser were preterm and this fraction did not vary substantially by ethnic group.