Serum gamma-glutamyltransferase (GGT) activity is a marker of oxidative stress and activity is associated with cardiovascular disease. Carotid intima-media thickness (IMT) is a noninvasive predictor of atherosclerosis. We investigated the association between serum GGT activity and carotid IMT. Fifty-five persons who had normal liver function tests were consecutively enrolled. Carotid IMT was evaluated in the right and left common carotid arteries. The averaged values of carotid IMT and serum GGT activity were compared. Serum GGT activity correlated with carotid IMT (r = .396, P = .003). Serum GGT activities were increased in patients with carotid intimal hyperplasia compared with those without intimal hyperplasia (20.3 +/- 11.2 vs 34.3 +/- 16.1 U/L; P = .001). Serum GGT activity is associated with carotid IMT. This finding supports the concept that elevated serum GGT activity is a marker of atherosclerosis.