Differences in mean birth weight and low birth weight (less than 2.5 kg) are analyzed among Asians, blacks, Hispanics, and whites who were enrolled in the Northern California Kaiser-Permanente Birth Defects Study, a prospective study of 29,415 pregnancy outcomes. Large differences in birth weight among babies of different ethnic groups persist after controlling for the joint effects of maternal smoking and alcohol use during pregnancy, sex of child, parity, length of prenatal care, and maternal weight-for-height percentile. Compared with whites, the relative mean differences in birth weight are estimated as -246 g for blacks, -210 g for Asians, -105 g for Hispanics, and -140 g for others. The low-birth-weight rates and crude odds ratios are 7.70% (2.17) for blacks; 5.57% (1.57) for Asians; 5.52% (1.55) for others; 4.00% (1.13) for Hispanics; and 3.55% (1.00) for whites. After controlling for the effects of 22 factors, the odds ratios for a low-birth-weight infant are 2.41 for blacks, 1.37 for Asians, 1.93 for others, and 1.25 for Hispanics. It is concluded that factors currently used to control for ethnic differences in birth weight are insufficient to explain the observed differences.