We investigated whether plasma levels of adiponectin in patients with both coronary artery disease (CAD) and diabetes mellitus (DM) are lower than in patients with CAD alone. We examined plasma adiponectin levels in 113 patients, 82 with CAD (40 of whom had both CAD and type 2 DM) and 31 normal controls. We found differences in plasma adiponectin levels between CAD patients with and without DM (7.8 ± 4.75 versus 12.1 ± 6.87 μg/mL, P = 0.002), between patients with CAD and controls (10.0 ± 6.27 versus 15.3 ± 5.38 μg/mL, P < 0.0001), and between men and women (10.2 ± 6.41 versus 13.1 ± 6.22 μg/mL, P = 0.017). Plasma adiponectin levels were correlated negatively with body mass index, triglyceride, total cholesterol, hemoglobin A1c, and fibrinogen levels (r = -0.456, P < 0.0001 ; r = -0.355, P < 0.0001 ; r = -0.286, P = 0.002; r = -0.299, P < 0.0001 ; r = -0.400, P < 0.0001, respectively), but were not significantly correlated with high sensitivity C-reactive protein or low density lipoprotein levels (r = -0.088, P = 0.352; r = -0.167, P = 0.077, respectively). Plasma adiponectin levels correlated positively with high density lipoprotein levels (r = 0.410, P < 0.0001). Our study demonstrates that plasma adiponectin levels in patients with both CAD and DM are lower than in patients with CAD alone. We speculate that people who have very low plasma adiponectin levels may be at increased risk of developing both CAD and DM.