Mortality of 91,540 nonsmoking wives was studied in relation to the smoking habits of their husbands by means of a cohort study in Japan. During 16 years of follow-up, 200 deaths from lung cancer took place. The relative risks of lung cancer in these nonsmoking wives were 1.00, 1.36, 1.42, 1.58, and 1.91 when husbands were nonsmokers, ex-smokers, or daily smokers of 1-14, 15-19, or 20 or more cigarettes daily, respectively. Specificity of association and internal consistencies were observed. Among cancers of each site, a similar tendency toward risk elevation in nonsmoking wives with smoking husbands was observed for nasal sinus cancer, brain tumors, and cancer of all sites besides lung cancer. In interpreting these results, the significance of proximity in exposure to sidestream smoke in Japanese homes was stressed.