Endometriosis is traditionally defined as the presence of endometrial glands and stroma in ectopic locations, especially the pelvic peritoneum, ovaries and rectovaginal septum. YKL-40, a new biomarker of inflammation, is secreted by activated macrophages and neutrophils in different tissues with inflammation. Serum concentrations of YKL-40 are elevated in patients with diseases characterized by inflammation. We aimed to investigate the possible association between serum YKL-40 levels and endometriosis. A total number of 88 women were recruited for this case-control study. About 53 patients with surgically proven endometriosis were included, while 35 patients without endometriosis comprised the control group. Patients were classified as having minimal, mild, moderate and severe disease in accordance with the severity. Two new groups were formed by combining patients with minimal and mild disease (Stage 1-2) and with moderate and severe disease (Stage 3-4). Serum YKL-40 levels were statistically higher in the endometriotic group compared to control group (p:0.001). YKL-40 levels were significantly higher in Stage 3-4 group compared to Stage 1-2 group (p values 0.001) as well. Correlation analysis revealed a positive correlation between serum YKL-40 levels and the stage of the disease. YKL-40 may be utilized as a marker for determining the severity of endometriosis.