Racial differences in hematocrit levels and the relationship between low and high hematocrit, intrauterine growth retardation, and preterm delivery were studied in a population of 17,149 low-income, iron- and folate-supplemented pregnant women. Hematocrit levels drawn during four gestational periods were considered. Evaluating pregnancy outcome in regard to hematocrit values at specific gestational ages eliminated the bias resulting in the previously reported association between a low hematocrit and preterm delivery in blacks. In contrast, in this study, in blacks, hematocrits of 27% to 30% were associated with lower but not significant reductions in the rates of intrauterine growth retardation and preterm delivery. Our major finding was that at 31 to 34 weeks, hematocrits > or = 40% were associated with significantly higher odds ratios for intrauterine growth retardation for both blacks and whites. These findings should prompt more attention to women who have high hematocrits in pregnancy while reducing concern for women of either race with low hematocrits.