Results of cross-sectional studies suggest an association between body mass index and asthma. However, it is not clear whether the occurrence of asthma precedes increased body mass index or vice versa. From 1971 to 1975, the First National Health and Nutrition Examination Survey collected height and weight data and information about doctor-diagnosed asthma from 14,407 subjects aged 25-74. In 1982 through 1985, information was again obtained on doctor-diagnosed asthma with a follow-up rate of 84.8%. We took this opportunity to examine the relationship between body mass index (BMI) and asthma in this cohort. Subjects with subnormal BMI and subjects admitting current or history of doctor-diagnosed asthma were excluded from the cohort. Mean follow-up was 10 years (range 6.7-13 years). Analyses were adjusted for race and gender. Logistic regression analysis was conducted with asthma as the dependent variable and BMI modeled as a categorical independent variable (BMI groups). At baseline and at follow-up, increasing BMI was associated with increased prevalence of asthma. During the observation interval, however, no increased incidence of asthma associated with increasing BMI was noted. In comparison with normal BMI, the relative risk (RR) for development of doctor-diagnosed asthma in elevated BMI was 1.0 (95% confidence interval 0.9-1.2), for markedly elevated BMI was 1.0 (0.8-1.3), and for severely elevated BMI was 1.1 (0.8-1.5). Race did not affect this relationship. African Americans had an increased risk of asthma, but the risk was unassociated with increasing BMI. Gender did not affect this relationship. The disease burden of asthma appeared in normal weight and slightly overweight women rather than obese and markedly obese women. These results suggest that asthma development may be a point on the trajectory of chronic obesity disease or asthma appears with obesity as a concurrent disorder.