A private census of Washington County, Maryland, in 1963 obtained information on smoking habits of all adults in the census, and death certificates of all residents who died in the next 12 years were coded for underlying cause of death and matched to the census. Among the white population aged 25 and over, 4,162 men and 14,873 women had never smoked. In this group, death rates from arteriosclerotic heart disease were significantly higher among men (relative risk (RR) = 1.31, 95% confidence interval (CI) 1.1-1.6) and women (RR = 1.24, 95% CI 1.1-1.4) who lived with smokers in 1963, after adjustment for age, marital status, years of schooling, and quality of housing. Among women, the relative risk increased significantly (p less than 0.005) with increasing level of exposure; among men, there was little evidence of a dose-response relation. The relative risks for nonsmokers who lived with smokers were greatest among both men and women who were younger than age 45 in 1963, but the number of deaths in these groups was small, and confidence intervals were broad. These results suggest a small but measurable risk for arteriosclerotic heart disease among nonsmokers who live with smokers.