An increased serum interleukin-6 (IL-6) level is associated with an increased risk of cardiovascular events in healthy subjects. However, it is unknown whether the level of serum IL-6 or genetic IL-6 polymorphism is correlated with the complexity of coronary plaque in patients with stable coronary artery disease (CAD). Patients with stable CAD (n = 135) were divided into 3 groups: insignificant coronary plaque (n = 77), simple coronary plaque (n = 15), and complex coronary plaque (n = 43). IL-6-174G > C polymorphism and serum levels of IL-6 and C-reactive protein (CRP) were investigated. No significant difference in the distribution of IL-6 genotypes was found among the groups. The presence of complex coronary plaque was associated with higher serum concentrations of IL-6 (P = 0.026) and CRP (P < 0.0001). To predict the presence of complex lesions, IL-6 > 5.8 ng/L and CRP > 2.6 mg/L had sensitivities of 86% and 74%, and specificities of 61% and 62%, respectively. By multivariate analysis, IL-6 > 5.8 ng/L and CRP > 2.6 mg/L were independently related to the presence of complex coronary plaque (P = 0.0002 and 0.004, respectively). IL-6 > 5.8 ng/L and CRP > 2.6 mg/L were associated with a 4.5-fold increase in the odds of having complex coronary plaque (P < 0.005). A simple measurement of the serum IL-6 level in patients with CAD can potentially identify subjects with complex coronary lesions and provide the option of aggressive medical strategies in a clinical setting.