Aims/Methods: To investigate whether smoking affects the serum level of leptin, 708 male workers aged 25–65 years old were cross-sectionally surveyed. Results: Multiple regression analysis indicated that among the various parameters examined, the level of leptin was positively associated with the body mass index and the levels of insulin, total cholesterol and uric acid, and was inversely associated with physical activity and the level of creatinine. The partial correlation coefficient of leptin was highest against the body mass index (r = 0.40), followed by insulin (0.29) and physical activity (–0.14), after adjustment for other leptin-related variables. However, no association was observed between the level of leptin and smoking (0.05), alcohol consumption (0.09) or age (0.09). Conclusions: The findings suggest that among life-style habits, physical activity, but not smoking or alcohol consumption, significantly affects the serum level of leptin in Japanese men.