OBJECTIVE YKL-40 is a novel inflammatory serum protein shown to be associated with the presence and prognosis of several malignancies. However, its prognostic relevance has not yet been analyzed in bladder cancer (BC). Therefore, the aim of this study was to assess the tissue, serum, and urinary levels of YKL-40 and their prognostic value in BC. METHODS YKL-40 gene expression levels were analyzed in frozen tissue samples of 91 patients with BC; YKL-40 concentrations were measured in 120 serum (101 patients with BC and 19 controls) and 154 urine samples (125 patients with BC and 29 controls). In 16 cases, corresponding serum samples collected before and after radical cystectomy were analyzed for YKL-40. Results were correlated with clinicopathological parameters and follow-up data. RESULTS YKL-40 gene expressions and serum concentrations were higher in patients with BC compared with controls; however, urinary YKL-40 levels remained under the detection limit in both patients and controls. Higher tissue gene expressions and serum concentrations were associated with poor patients' survival in the univariable analysis (P = 0.037 and 0.022, respectively), but only high YKL-40 serum levels proved to be independent prognostic factors in BC (hazard ratio = 1.755, 95% CI: 1.014-3.039, P = 0.045). We found no significant difference between preoperative and postoperative serum concentrations of YKL-40. CONCLUSIONS YKL-40 serum levels are associated with the presence of BC and poor patients' survival. The independent prognostic relevance of YKL-40 is of particular interest in patients with muscle-invasive BC treated with radical surgery. Our data suggest that BC tissue is not the main source of serum YKL-40 levels.