Introduction:
Both smoking and diabetes are known to have health risks, but the extent of the risks when both co-existed are little known.

Methods:
A cohort, consisting of 422,771 adults, was recruited successively during health surveillance between 1996 and 2008. Data from questionnaire, and results from fasting blood and other screening tests were collected. Hazard ratios (HR) came from Cox model and life expectancy from life table method.

Results:
One quarter of the cohort were current smokers (23.2%), but only 7% had pre-diabetes and 6% diabetes. HR for all-cause mortality for smoking, 1.51, for diabetes, 2.02, but when co-existed, 3.38. Life span shortened 6 years for smokers, 9 years for diabetes, but 15 years for combined. HR for all cancer increased from 1.77 (smoking) or 1.42 (diabetes) to 2.87 (combined), for heart disease, from 1.43 (smoking) or 2.24 (diabetes) to 4.05 (combined), and for stroke, from 1.54 (smoking) or 1.82 (diabetes) to 2.69 (combined).

Conclusions:
Smokers with diabetes became doubly deadly for all cause (1.51 to 3.38), all cancer (1.77 to 2.87) and heart disease (1.43 to 4.09) and shortened life expectancy by 15 years, from 6 years of smoking and 9 years of diabetes. Overwhelming majority of smokers were not aware of such a large risk. The large life expectancy risk could be leveraged to motivate smokers into smoking cessation.