The US Surgeon General has concluded that there is no risk-free level of secondhand smoke (SHS) exposure. While the physical consequences of such exposure are well established, the potential mental health effects of SHS on children and adolescents are still unclear. Data on children aged 8 to 15 years from the 2001 to 2004 National Health and Nutrition Examination Survey were used to assess the effects of SHS exposure, as measured by serum cotinine level, on symptoms of mental disorders. Exposure to SHS was positively associated with Diagnostic and Statistical Manual of Mental Disorders (Fourth Edition) symptoms of major depressive disorder, generalized anxiety disorder, attention-deficit/hyperactive disorder, and conduct disorder among the total sample of nonsmokers, even when adjusting for age, sex, race/ethnicity, poverty, asthma, hay fever, migraine, maternal smoking during pregnancy, and allostatic load. Given the critical developmental period of childhood and adolescence, the effects of policy to reduce or ban smoking in public places and in the home may help prevent or reduce the progression of illness in at-risk individuals and alleviate the heavy burden of illness attributable not only to tobacco but also to mental disorders.