To evaluate the impact of body mass index (BMI) (weight (kg)/height (m)(2)) and change in body silhouette on asthma risk, the authors investigated a cohort of women participating in the E3N Cohort Study in France from 1990 to 1993. The authors identified 372 incident cases of asthma among 67229 women aged 40-65 years at baseline. Data were analyzed using proportional hazard models to determine the risk of asthma incidence in relation to BMI and body silhouette. Reported BMI at baseline was related to the incidence of asthma, with a significantly increased risk corresponding to increasing BMI quintiles (test for trend: p < 0.001). Compared with a BMI of 20-21.4 (second quintile), women with a BMI >or=27 had double the risk of incident asthma after adjustment for potentially confounding factors (multivariate relative risk = 2.02, 95% confidence interval: 1.38, 2.98). Increase in body mass between menarche and the start of the follow-up study was significantly related to asthma risk (for an increase of two silhouettes, relative risk = 1.66, 95% confidence interval: 1.18, 2.32). The authors conclude that a high BMI is significantly related to the risk of asthma incidence and that an increase in body silhouette between menarche and adulthood is related to the incidence of asthma later in life.