To estimate the diagnostic and prognostic value, pathological and clinical correlation of cancer antigen 125 (CA125) in ovarian cancer (OC). Retrospective analysis was done of 350 patients who were operated for OC in years 1990-2001 in Gynecology Clinic MU of Gdansk. We analyzed those before primary operation (PO) and second look laparotomy (SLL). Chi 2 and t-Student tests were used.Before PO 18% OC patients had CA125 less than 35 and 43.8% more than 600 U/ml, for benign tumors it was 59.9 and 1.1 respectively (p 600 before SLL and all of those had progress disease. The positive and negative predictive value of CA125 before SLL were 0.94 and 0.56 respectively. Cytoreduction with no macroscopic disease was achieved in 45% of patients with CA125 600 (p = 0.001). We looked for differences of CA125 levels depending on clinical and pathological data. According to our results only histology (p = 0.02) and clinical stage (p = 0.02) influenced CA 125 levels.There is a good correlation between elevated levels of CA125 and state of the disease in SLL, and we consider SLL as obligatory to perform as there is a low negative predictive value of CA125. The CA125 before primary operation has prognostic significance to possibility of optimal cytoreduction.