We examined whether serum gamma GTP activity (gamma GTP) is associated with Breslow's lifestyle index and whether it could be used as a tool to detect subjects with unhealthy lifestyles. To that effect, 724 male Japanese workers excluding patients suffering from hepatitis virus infection, autoimmune liver diseases and apparently active bile duct diseases were cross-sectionally examined. gamma GTP was inversely associated with the total score of Breslow's index for all subjects (lambda=30.643) and in subjects aged 40 or more (lambda=37.073). The association was consistent even after adjustments of subjects' ages and levels of triglycerides, total cholesterol and uric acid (p=0.0001). Among the seven lifestyle factors comprising Breslow's index, improper habits of drinking (p < 0.0001), smoking (p=0.0204), exercise (p=0.0189) and body weight control (p < 0.0001), were associated with increased gamma GTP. Even in a survey in which subjects who had proper habits of drinking and body weight control were selectively examined, improper habits of smoking and exercise still tended to be associated with increased gamma GTP. Receiver operating characteristic curves indicated that gamma GTP was beneficial for detecting subjects who scored two or less on Breslow's index, at least in subjects aged 40 or more. gamma GTP was associated with insulin resistance level estimated by the homeostasis model assessment (p < 0.0001), which was inversely associated with Breslow's index (p=0.0040). gamma GTP could be used as an objective substitute of Breslow's index, allowing us to identify subjects with low scores on Breslow's index, at least after sorting subjects properly. Such screening would enable interventions to correct subjects' unhealthy lifestyles, helping to solve lifestyle-related disease issues.