Reasons for the persistent difference in rates of preterm delivery among black and white women are not clear. Known risk factors explain very little of the variance. Recent studies have shown that social class does not fully account for poor pregnancy outcomes among black women. Cultural and environmental factors that vary between the races, but not between the different socioeconomic levels within a race, may account for some of the unexplained ethnic differences in preterm delivery. Any potentially negative exposure that is distributed differentially between racial groups warrants particular attention. The major hypothesis of this research is that US black women are chronically exposed to specific stressors that adversely affect the outcomes of their pregnancies. A psychosocial stress model has been proposed to explain the complex interactions of social, environmental, and medical factors that are unique among women of color. To generate data for the stress model, a research strategy has been designed to identify psychosocial and behavioral risk factors that have a physiologic impact on pregnancy outcome. We propose that race is a marker for this stress but is not in itself a risk factor for preterm delivery.