An assessment of scientific literature data on changes in the risk of developing cancer, cardiovascular disease (CVD), or chronic obstructive pulmonary disease (COPD) following smoking cessation was conducted by an international working group of experts convened at the International Agency for Research on Cancer (IARC), in Lyon, from March 13 to 19, 2006. Three questions were addressed: 1) Is the risk of developing cancer (for each of the 13 tobacco-related cancers considered), CVD, or COPD lower among former smokers than among current smokers who are otherwise similar in all other respects? 2) Among former smokers, is the reduction in the risk of developing a disease proportional to the duration of abstinence? 3) After a long period of abstinence, does the risk return to the level of that of people who have never smoked? The main results of this assessment are presented in this article. In summary, a significant benefit of smoking cessation, increasing with the duration of abstinence, was observed for all major cancers associated with smoking, but it only rarely reaches the risk level of people who have never smoked. Quitting smoking reduces the excess risk of dying from cancer, CVD, or COPD linked to continued smoking. These data can be used to support public policies encouraging smoking cessation. (R.A.)