YKL-40 is a new biomarker in serum with a prognostic value in several localized and metastatic malignancies. The current knowledge regarding the biological functions of YKL-40 in cancer links YKL-40 to increased aggressiveness of the tumor. Utilizing tissue microarrays, YKL-40 protein expression in tumor tissue was assessed by immunohistochemistry in a cohort of 630 high-risk breast cancer patients with a median estimated potential follow-up time of 10 and 13 years for disease-free (DFS) and overall survival (OS), respectively. YKL-40 protein expression was found in malignant tumor cells and in inflammatory cells. High expression was associated with positive estrogen and progesterone receptor status and high tumor differentiation. Contrary to studies on serum YKL-40 as a prognostic biomarker, a high YKL-40 expression in tumor cells was not significantly associated with DSF and OS in univariate and multivariate analyses.