Background The inflammatory biomarker YKL-40 has previously been studied as a potential risk marker in cardiovascular disease. We aimed to assess the prognostic reclassification potential of serum YKL-40 in patients with stable coronary artery disease. Methods and Results The main study population was the placebo group of the CLARICOR (Effect of Clarithromycin on Mortality and Morbidity in Patients With Ischemic Heart Disease) trial. The primary outcome was a composite of acute myocardial infarction, unstable angina pectoris, cerebrovascular disease, and all-cause mortality. We used Cox proportional hazards regression models adjusted for C-reactive protein level and baseline cardiovascular risk factors. Improvement in prediction by adding serum YKL-40 to the risk factors was calculated using the Cox-Breslow method and c-statistic. A total of 2200 patients were randomized to placebo, with a follow-up duration of 10 years. YKL-40 was associated with an increased risk of the composite outcome (hazard ratio per unit increase in (YKL-40) 1.13, 95% CI 1.03-1.24, P=0.013) and all-cause mortality (hazard ratio 1.32, 95% CI 1.17-1.49, P<0.0001). Considering whether a composite-outcome event was more likely to have, or not have, occurred to date, we found 68.4% of such predictions to be correct when based on the standard predictors, and 68.5% when serum YKL-40 was added as a predictor. Equivalent results were obtained with c-statistics. Conclusions Higher serum YKL-40 was independently associated with an increased risk of adverse cardiovascular outcomes and mortality. Addition of YKL-40 did not improve risk prediction in patients with stable coronary artery disease. Clinical Trial Registration URL: https://www.clinicaltrials.gov/. Unique identifier: NCT00121550.