YKL-40 (human cartilage glycoprotein-39) is a member of family 18 glycosyl hydrolases. YKL-40 is a growth factor and is secreted by cancer cells. High serum levels of YKL-40 in patients with colorectal cancer and recurrent metastatic breast cancer have been associated with a poor prognosis. We evaluated the prognostic value of plasma YKL-40 in patients with primary ovarian cancer (OC). YKL-40 was determined by ELISA in plasma obtained preoperatively from 47 women with stage III OC and in plasma from 79 healthy females. The results showed that plasma YKL-40 was elevated compared to healthy females in 57% of the OC patients and was highest in the patients who died during the follow-up compared to the patients still alive (186 vs. 78 micro g/l, p=0.002). Patients with high plasma YKL-40 (>130 micro g/l) had significantly (p=0.0003) shorter survival than patients with normal plasma YKL-40. Multivariate Cox regression analysis showed that plasma YKL-40 (RH=3.95; 95% CI, 1.52-10.27; p=0.005) and radicality after primary surgery (RH=4.03; 95% CI, 1.81-8.97; p=0.001) were independent prognostic factors of survival, whereas age, histological type of tumour and serum CA125 had no independent prognostic value. In conclusion, plasma levels of YKL-40 proved of prognostic value in stage III OC patients.