YKL-40, also called human cartilage glycoprotein-39 is homologs of family 18 glycosyl hydrolases secreted by human macrophages. Although high levels of YKL-40 is associated with several diseases. YKL-40, a growth factor for connective tissue cells, a migration factor for endothelial and vascular smooth muscle cells, is expressed by several types of solid human carcinoma, including prostate carcinoma.The aim of this study was to evaluate diagnostic role of serum YKL-40 levels in primary prostate cancer and detection of recurrences after radiotherapy.Methods: YKL-40 determined in serum samples from 50 patients with primary prostate cancer and 25patients with benign prostatic hyperplasia as control. Serum YKL-40 levels were measured by ELISA. PSA levels were also measured by using IMMULIT system.Results: Serum YKL-40 levels were significantly higher (P= 0.000) in patients with prostate cancer compared with control groupwhereas no significant elevation in BPH. Conclusion: High serum YKL-40 levels in patients with primary prostate cancer indicate that YKL-40 may have a function in theProgression of malignant diseases, whereas no significant elevation was observed in benign prostatic hyperplasia. Further studies are needed to elucidate the biologic role of YKL-40 in cancer aggressiveness and in progression of malignantdiseases.