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We examined whether secondhand smoke exposure during childhood was associated with cancer mortality in adulthood among never smokers. In the Japan Collaborative Cohort Study for Evaluation of Cancer Risk, we analyzed data from 45,722 Japanese lifetime nonsmokers aged 40–79 years with no history of cancer at baseline (1988–1990) who had completed a lifestyle questionnaire, including information on the number of family members who had smoked at home during their childhood (0, 1, 2, or ≥3 family members). A Cox proportional hazards model and competing-risks regression were used to calculate multivariable hazard ratios and subdistribution hazard ratios with 95% confidence intervals for overall and site-specific cancer mortality according to the number of family members who smoked during the participant's childhood, after adjusting for potentially confounding factors. During a median follow-up period of 19.2 years, a total of 2,356 cancer deaths were documented. Secondhand smoke exposure was positively associated with the risk of mortality from pancreatic cancer in adulthood; the multivariable hazard ratio for having 3 or more family members who smoked (as compared with none) was 2.32 (95% confidence interval: 1.14, 4.72). Associations were not evident for total cancer risk or risk of other types of smoking-related cancer. In this study, secondhand smoke exposure during childhood was associated with an increased risk of pancreatic cancer mortality in adulthood. [ABSTRACT FROM AUTHOR]