The authors estimated the association between asthma and body mass index in a 1963-2002 study of 135,000 Norwegians aged 14-60 years who were followed on average for 21 years. Cox proportional hazards regression models were fitted to estimate the relative risk of asthma adjusting for smoking, education, and physical activity. Compared with persons with a body mass index (weight (kg)/height (m)(2)) of less than 25, overweight (body mass index: 25-29) men and women had relative risks of asthma of 1.27 (95% confidence interval (CI): 1.13, 1.43) and 1.30 (95% CI: 1.17, 1.45), respectively, while obese (body mass index: >/=30) men and women had relative risks of 1.78 (95% CI: 1.35, 2.34) and 1.99 (95% CI: 1.67, 2.37), respectively. Stratified analyses revealed a similar association between body mass index and asthma for never smokers, ever smokers, persons with less than or equal to 12 years of education, and persons with more than 12 years of education. Analyses including all the covariates gave results similar to those not adjusting for these factors. The risk of asthma increased steadily with body mass index, from a body mass index of 20 in men and of 22 in women. In men, the risk of asthma increased by 10% with each unit of increased body mass index between 25 and 30. The similar value for women was 7%. Overweight or obese persons reported asthma more often than did thinner persons after adjustment for smoking, education, and physical activity.