In an effort to study a possible association between environmental tobacco smoke (ETS) exposure and heart disease, an analysis was conducted of a cohort of 353,180 never smoking women and 126,500 never smoking men enrolled in the American Cancer Society's Cancer Prevention Study-II. The analysis focused on subcohorts of 309,599 married pairs and of 135,237 subjects concordant for self reported exposure and exposure reported by each person's spouse. More than 2,800 coronary heart disease (CHD) deaths were noted among the married pairs. Of the married men, 10% were married to currently smoking spouses. This was also the case for 28% of the married women. A 22% higher CHD mortality was noted among never smoking men married to currently smoking wives compared with those married to women who had never smoked. Never smokers living with former smokers showed no increased risk. The findings pointed to a modestly elevated risk for heart disease (20%) due to ETS exposure among never smokers. The study was consistent with others in this field, was controlled for a number of known cardiovascular risk factors, and paralleled the data for active smoking being somewhat stronger in younger individuals and in showing a predominantly acute effect. The observed relative risks were small and could be due to confounding by unmeasured risk factors. Positive dose response relationships among the exposed were in general not present.