No level of exposure to secondhand smoke (SHS) is safe.1 Breathing SHS can cause heart disease and lung cancer in nonsmoking adults and increases the risk for sudden infant death syndrome, acute respiratory infections, middle-ear disease, and exacerbation of asthma in children.1- 3 In the United States, exposure to SHS declined approximately 70% from the late 1980s through 2002, most likely reflecting widespread implementation of laws and policies prohibiting smoking in indoor workplaces and public places during this period.1,4 Although the major sources of SHS exposure for nonsmoking adults are the home and workplace, the primary source of SHS exposure for children is the home1; therefore, eliminating smoking in workplaces and public places is less likely to reduce children's exposure to SHS. This report examines changes in the prevalence of self-reported SHS exposure at home and changes in any exposure, as measured by serum cotinine (a biologic indicator of SHS exposure), in nonsmoking children, adolescents, and adults. The analysis was conducted using data from the 1988-1994 and 1999-2004 National Health and Nutrition Examination Surveys (NHANES). The results indicated that self-reported SHS exposure at home and SHS exposure as measured by serum cotinine declined significantly (i.e., by 51.2% and 44.7%, respectively) in the U.S. population from 1988-1994 to 1999-2004; however, the decline was smaller for persons aged 4-11 years and 12-19 years. These results underscore the need to continue surveillance of SHS exposure and to focus on strategies to reduce children's SHS exposure.