Scarce information is available, particularly from Europe, on why smokers quit. We analyzed this issue in a large dataset of Italian ex-smokers. Six population-based surveys on smoking were annually conducted in 2005-2010 on a representative sample of the Italian adult population, which included more than 3000 participants each year. A specific question on the main reason for quitting smoking was answered by a total of 3075 ex-smokers (1936 men and 1139 women). Overall, 43.2% of ex-smokers mentioned a current health condition as the main reason to stop smoking, 31.9% stopped to avoid future health problems, 6.3% stopped because of pregnancy or child birth, 4.0% because of imposition by the partner/family, 3.7% because of a physician's recommendation, 3.0% because of the economic cost, 0.5% because of smoking bans, and 4.6% because of other reasons. Statistically significant differences in the motivation to quit smoking have been found according to sex, age, social class, and smoking history. The majority of ex-smokers quit because of tobacco-related health conditions. Only a minority of ex-smokers quit to avoid future illness. Physicians should be encouraged to assist smokers to quit. The current prices of cigarettes in Italy are not sufficiently high to discourage people from continuing smoking.