Objectives. This study examined whether heightened cardiovascular reactivity and low socioeconomic status had synergistic effects on the progression of carotid atherosclerosis in a population of eastern Finnish men. Methods. Data from the Kuopio Ischemic Heart Disease Risk Factor Study were used to measure 4-year progression of intima-media thickness in 882 men according to cardiovascular reactivity and socioeconomic status. Associations were examined in relation to risk factors and were stratified by base-line levels of atherosclerosis and prevalent ischemic heart disease. Results. The effect of reactivity on atherosclerotic progression depended on socioeconomic status. Men who had heightened cardiovascular responsiveness to stress and were born into poor families, received little education, or had low incomes had the greatest atherosclerotic progression. Conclusions. An understanding of associations between individual risk factors and disease should be based on etiologic hypotheses that are conceived at the population level and involve fundamental social and economic causes of disease. This study demonstrates how examining the interaction of an individual biological predisposition with low socioeconomic status over the life course is etiologically informative for understanding the progression of atherosclerotic vascular disease.