Purpose: We conducted this study because the duration of excess lung cancer risk among former smokers has been inconsistently reported, doubt has been raised regarding the population impact of smoking cessation, and differential risk reduction by histologic cell type after smoking cessation needs to be confirmed. Methods: The Iowa Women's Health Study is a prospective cohort study of 41,836 Iowa women aged 55 to 69 years. In 1986, mailed questionnaires were used to collect detailed smoking history. Age-adjusted lung cancer incidence through 1999 was analyzed according to years of smoking abstinence. Relative risks were estimated using Cox regression analysis. Results: There were 37,078 women in the analytic cohort. Compared with the never smokers, former smokers had an elevated lung cancer risk (relative risk, 6.6; 95% confidence interval, 5.0 to 8.7) up to 30 years after smoking cessation for all former smokers. However, a beneficial effect of smoking cessation was observed among recent and distant former smokers. The risk of adenocarcinoma remained elevated up to 30 years for both former heavier and former lighter smokers. Conclusion: The risk for lung cancer is increased for both current and former smokers compared with never smokers and declines for former smokers with increasing duration of abstinence. The decline in excess lung cancer risk among former smokers is prolonged compared with other studies, especially for adenocarcinoma and for heavy smokers, suggesting that more emphasis should be placed on smoking prevention and lung cancer chemoprevention.