Background Guideline for smoking cessation often mentioned physical activity as a desirable behavior but has never considered it essential. This study aims to quantity the short-term and long-term benefits of leisure-time physical activity (LTPA) among smokers, including its impact on quitting behavior and on mortality outcome. Methods This cohort, consisting of 434,189 individuals (23.2% smokers and 6.1% ex-smokers), went through standard medical surveillance program(s) from 1996 to 2008. History for smoking and physical activity was collected in the initial visit and subsequent visits. Exercise volume, expressed in MET·hour/week at each visit, classified each individual into inactive (<3.75), low-active (3.75-7.49), or fully active (≥7.50) group. Fully active individuals met the current recommendation of 30 minute/day for 5 days or more per week. Hazard ratios (HR) were calculated with Cox model for mortality, adjusted for 10 confounders. Life expectancy was calculated using life table method. Results One fifth of smokers (22.9%) and one third of former smokers (37.4%) were active, an under-appreciated phenomenon favoring exercisers for success in cessation. Smokers who were fully active increased quit rate by 55%-81%. Furthermore, active smokers had up to 30% less relapse rate when compared to less active individuals. Smokers who remained physically active had significant reduction in mortality for all-cause, all-cancers and cardiovascular diseases, by 23%, 14% and 34%, respectively. Moreover, they gained 3.7 years in life expectancy. Smokers who quit and stayed active reduced mortality by 43% and gained 5.6 years in life expectancy. [Level of exercise on allcause mortality by smoking] Conclusions To engage in physical activity has unexpected benefits for smokers. It improved the success rate for smoking cessation, reduced relapse rates and extended life expectancy. Physical activity should be included as an essential aspect in the current guideline for smoking cessation.