BACKGROUND: The link between socioeconomic status (SES) and health is widely recognized but the pathophysiologic mechanisms are not well understood. We tested the hypothesis that parasympathetic nervous system (PNS) regulation is one such mechanism. METHODS: In a cross-sectional study, electrocardiogram-derived RR interval variability (RRV), a non-invasive index of cardiac PNS regulation, and SES, measured as educational attainment and income, were collected in 756 subjects in the CARDIA study of heart disease in young adults. RESULTS: Relative to those with less than a high school education, those with high school to college and post-college education had 26% (beta = 0.233) and 43% (beta = 0.355) greater low frequency (LF) RRV, respectively, adjusted for age, sex, and race. For high frequency (HF) RRV, race interacted with income: relative to low income whites, intermediate and high income whites had 133 and 191% greater HF power, respectively, while intermediate and high income blacks had 32 and 44% greater HF RRV, respectively, relative to low income blacks. CONCLUSIONS: Numerous studies demonstrate that psychosocial stressors reduce cardiac parasympathetic regulation and that SES disparities are associated with increasing social stress proportional to the degree of disparity. Data from the current study suggest that PNS regulation may be a mechanism linking the stressful effects of low SES to increased morbidity and mortality.