To understand the significance of elevated serum gamma-GTP levels, factors relevant to the serum gamma-GTP level were studied using data of health check-ups for the employees of a Japanese corporation. The gamma-GTP level was positively correlated with levels of various liver function tests including aspartate aminotransferase (AST) and alanine aminotransferase (ALT). Except for them, the gamma-GTP level was positively correlated with levels of insulin resistance, uric acid, total cholesterol, triglyceride, and body mass index. The correlation between the gamma-GTP level and LDL cholesterol was also observed only when subjects who drank more than 5 times a week were selectively studied. When non-drinkers and opportunity drinkers were selectively studied, 63.6% of subjects whose gamma-GTP level was more than 120 IU/liter showed elevated insulin resistance levels. Multiple factors including insulin resistance may affect serum gamma-GTP activity in clinical subjects.