Article ID / Artikelinumero: 74. © 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons by Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/). ; The influence of parents’ smoking on children’s smoking is well known, but few studies have examined the association between grandparents’ and grandchildren’s smoking. We studied the association between paternal and maternal grandparents’ smoking and their grandchildren’s tobacco use and assessed whether parents’ smoking is a mediator in this process. Data were obtained from a national survey of 12–18-year-old Finns in 2013 (N = 3535, response rate 38%). Logistic regression and mediation analyses were used. Both boys and girls had higher odds for smoking experimentation, daily smoking and other tobacco or tobacco-like product use if their mother, father or any of the four grandparents were current or former smokers. When parents’ and grandparents’ smoking status were included in the same model, grandparents’ smoking generally lost statistical significance. In the mediation analysis, 73% of the total effect of grandparents’ smoking on grandchildren’s daily smoking was mediated through parents’ smoking, 64% on smoking experimentation and 63% on other tobacco or tobacco-like product use. The indirect effect of a mother’s smoking was higher than that of a father’s. To conclude, paternal and maternal grandparents’ smoking increases grandchildren’s tobacco use. The influence is mainly, but not completely, mediated through parents’ smoking.