The purpose of this study was to determine the prevalence and source of passive smoke exposure among children with chronic respiratory diseases and compare these to both a well child and nonrespiratory chronic illness child population. Rates and source of passive smoke exposure were compared among four child groups: asthma, cystic fibrosis, rheumatoid arthritis, and well children using a questionnaire mailed to the parents of the selected children. Twenty percent of respondents reported current smoking with a significantly higher rate among the cystic fibrosis and rheumatoid arthritis groups. One-third of all children surveyed were exposed to passive smoke at home and/or day care on a daily basis. Over 80% of the asthma and cystic fibrosis respondents reported a change in smoking behavior (i.e., smoking outside the home or smoking fewer cigarettes) after the diagnosis of their child's illness as compared with only 40% of the nonrespiratory groups. Health care providers need to inquire about potential sources of passive smoke exposure in their patients, particularly children with chronic respiratory disease.