Objectives. We investigated the relationship between secondhand smoke and periodontal disease in nonsmokers., Methods. We undertook a cross-sectional analysis of the Atherosclerosis Risk in Communities study with 2739 lifetime nonsmokers aged 53–74 years, unexposed to other sources of tobacco, who received a complete periodontal examination at visit 4. Exposure was reported as average hours per week in close contact with a smoker in the preceding year. We defined severe periodontitis as 5 or more periodontal sites with probing pocket depth of 5 millimeters or more and clinical attachment levels of 3 millimeters or more in those sites. Other outcomes were extent of periodontal probing depths of 4 millimeters or more and extent of clinical attachment levels of 3 millimeters or more., Results. In a binary logistic regression model, adjusted odds of severe periodontitis for those exposed to secondhand smoke 1 to 25 hours per week increased 29% (95% confidence interval = 1.0, 1.7); for those exposed to secondhand smoke 26 hours per week, the odds were twice as high (95% confidence interval = 1.2, 3.4) as for those who were unexposed., Conclusions. Exposure to secondhand smoke and severe periodontitis among nonsmokers had a dose-dependent relationship.