Abstract Objectives: The aim of our study was to assess the prognostic role of CA 125 regression during neoadjuvant chemotherapy (NAC) in patients with ovarian cancer (OC) or primary peritoneal serous carcinoma (PPSC) that underwent interval debulking surgery (IOC). Material and methods: Thirty one patients with advanced OC or PPSC (FIGO stage IIIC and IV) who underwent initial exploratory surgery, followed by NAC containing platinum analogs, have been analyzed, retrospectively. We have used a regression coefficient (RCA 125), which was calculated as following: log10 (CA 125 level measured after two cycles of NAC/ baseline CA 125) for statistical analysis. The median value of RCA 125 reached -0.788 and has been used to dichotomize. Optimal IOC has been performed in 67.74% (21/31) patients, suboptimal in 25.81% (8/31) patients and 6.45% (2/31) of patients did not undergo IOC due to the progression of the disease. Results: We have noted significant correspondence between time to progression and RCA 125 in univariate analysis, which we have also confirmed in multivariate analysis (HR 0.27; 95%CI, 0.15-0.96; p=0.0118). Similarly, we have observed significant relationship between overall survival, RCA 125 and extension IOC in univariate analysis. Multivariate analysis confirmed that RCA 125 was independent prognostic factor, HR-0.18 (95%CI, 0.07-0.56; p=0.004). In case of patients with high RCA 125, a greater rate of optimal debulking cytoreduction (p=0.0278, U=50.0) has been observed. Conclusions: RCA 125 after two courses of NAC appears to be an important prognostic factor in patients with OC or PPSC, who underwent IOC. High RCA125 during NAC seems to be a good predictive factor in order to achieve optimal IOC.