To assess smoking, obesity, and other risk factors for asthma, the authors examined 17,605 subjects aged 12 years or more who participated in the National Population Health Survey in 1994-1995. Asthma was considered present if an affirmative response was given to the question, "Do you have asthma diagnosed by a health professional?" The authors used analytic weights incorporating a design effect to take the complex survey design into account. The prevalence of asthma was 10.4% for males and 11.2% for females aged 12-24 years. Among the subjects aged 25 years or more, the prevalence varied from 4.1% to 5.8% for men and from 4.9% to 6.4% for women. Female smokers demonstrated a 1.7-fold increase in the prevalence of asthma compared with female nonsmokers, with the smoking effect more pronounced among female children and young adults. In contrast, there was no significant relation between smoking and asthma in males. The prevalence of asthma increased with increasing body mass index in females, but not in males. Immigrant status, history of allergy, and household income were significant predictors for both genders. Low household income was associated with a higher prevalence of asthma in men and women.