The relationship between γ-glutamyltransferase (γ-GTP) and C-reactive protein (CRP) has not been established, particularly in the lean or non-overweight population. In the present study, we examined the associations between γ-GTP and CRP in non-overweight and overweight middle-aged Japanese men. We conducted a cross-sectional study of 4,271 apparently healthy men aged 40 to 64 y (mean±SD, 50.5±6.6 y) who participated in health checkups. Associations between serum CRP levels, other clinical parameters, and lifestyle factors were analyzed using Spearman's rank correlation coefficient analysis and multiple linear regression analysis in the non-overweight (body mass index [BMI]<25 kg/m(2)) and overweight (BMI≥25 kg/m(2)) men. Associations between serum γ-GTP activity and serum CRP levels were analyzed using analysis of covariance by comparisons of serum CRP levels of four subgroups according to γ-GTP status. In non-overweight men, BMI, high-density lipoprotein cholesterol, triacylglycerols, fasting blood glucose, aspartate aminotransferase, γ-GTP, and smoking habit were positively associated with serum CRP levels. In overweight men, BMI, diastolic blood pressure, triacylglycerols, and γ-GTP were positively associated with serum CRP levels. After adjustment for age, BMI, smoking status, and alcohol intake, dose-response relationships were observed between γ-GTP and CRP levels in both overweight and non-overweight men. The results of this study indicate that an increase in serum γ-GTP activity is closely associated with elevated CRP levels in both non-overweight and overweight middle-aged Japanese men.