Summary. Smoking may be a risk factor for diabetes, and it has been suggested that viral hepatitis may predispose to diabetes. We studied diabetes and smoking histories in people with viral hepatitis. From 1997 to 2004, we studied the risk of incident diabetes in a community cohort with hyperendemic HBV and HCV infection in southern Taiwan. The cohort involved 3539 people (40-70 years old) without diabetes. Four hundred and twenty-three individuals developed diabetes. Those who were >/=65 years old, frequently consumed alcohol, had a BMI >/=25, had <9 years of education, were anti-HCV(+) or smoked >/=1 pack per day were more likely to develop diabetes (P < 0.05). A cumulative hazard function test showed that the higher the smoking levels, the greater the cumulative incidence rate of diabetes in HBsAg(+) participants only (P = 0.03 by log-rank test). A multiple Cox proportional hazards model analysis in different hepatitis statuses showed smoking levels were strong predictors of diabetes with a dose-response relationship for type 2 diabetes in those with HBsAg(+): hazard ratio (HR) = 3.8, (95% CI: 1.2, 12.3) for light smokers (<1 pack per day) and HR = 4.4 (95% CI: 1.5, 13.3) for heavy smokers (>/=1 pack per day). Increasing BMI was a common predictor in all people. Smoking is a strong predictor for diabetes with a dose-response relationship in HBsAg(+) individuals and a mild predictor for seronegative individuals but not significant in anti-HCV(+) individuals.