Despite recognition of the deleterious effects of passive smoking, quantitative information on the intake of environmental tobacco smoke is still lacking. Cotinine is the major metabolite of nicotine found in the urine. We have examined the relationship between urinary cotinine excretion in 472 nonsmokers and the smokiness of their environment. The urinary cotinine levels of nonsmokers who lived with smokers were higher than those of nonsmokers who did not, increasing with the combined daily cigarette consumption of smokers in the family. The urinary cotinine values of nonsmokers who worked with smokers were also higher than those of nonsmokers who did not, increasing with the number of smokers in the workroom. The presence of smokers in both the home and the workplace also increased the cotinine levels. Urban nonsmokers had more cotinine in their urine than rural nonsmokers. We conclude that the deleterious effects of passive smoking may occur in proportion to the exposure of nonsmokers to smokers in the home, the workplace, and the community.