Study objective: To examine the relation of smoking habits and development of asthma in a large cohort of US women.

Design: Prospective cohort study.

Participants: Among 74,072 women, 34 to 68 years of age, who were free of major diseases, we documented 671 incident asthma cases and 798 incident cases of chronic bronchitis during 10 years of follow-up.

Methods: Age-adjusted relative risk estimates for smoking categories were calculated separately for chronic bronchitis and asthma.

Results: Risk of chronic bronchitis was significantly higher in current smokers than in never smokers (relative risk [RR] = 2.85; 95% confidence interval [CI] = 2.45 to 3.32) and increased with the number of cigarettes smoked per day (p for trend < 0.0001). Approximately 5 years after quitting, chronic bronchitis risk in past smokers approached that in never smokers. In contrast, current smokers were at significantly lower risk for asthma than women who never smoked (RR = 0.57; 95% CI = 0.46 to 0.71) and women who quit (RR = 0.50; 95% CI = 0.40 to 0.62), possibly because individuals with sensitive airways are less likely to become regular smokers, and smokers who develop respiratory symptoms of any etiology tend to quit. Asthma risk in past smokers initially increased compared with that in never smokers, possibly because of quitting prior to diagnosis in response to symptoms of any etiology, but decreased with time since quitting (p for trend = 0.007); within approximately 5 years, the risk did not differ between past and never smokers.

Conclusion: These data suggest that smoking in adults may not be an independent cause of asthma but could exacerbate or be perceived as exacerbating asthma symptoms in susceptible individuals.