The associations of the CA125 regression rate with initial response to chemoth erapy and prognosis remain unclear. We examined the association between CA125 re gression in neoadjuvant chemotherapy (NAC) and prognosis. Fifty patients with ad vanced ovarian cancer (TNM classification TIIIc or M1) who received initial NAC and did not undergo significant cytoreductive surgery were selected for the retr ospective analysis, after excluding clear cell carcinoma and mucinous adenocarci noma putative to be cisplatin-resistant. For each patient, regression coefficie nt was calculated using all the CA125 levels measured from the day of NAC as day 0 until the day of normalization of CA125 level (< 35 IU/ml) or the day of stan dard surgery. Responder was defined as a regression coefficient of -0.039 or gr eater (33 cases) and nonresponder as a regression coefficient less than -0.039 (17 cases). The 3-year survival rate for all 50 cases was 59.3%. When stratifi ed by regression coefficient of CA125 levels, the 3-year survival was 70.5%in responders and 43.3%in nonresponders. Univariate analysis identified the regres sion coefficient of CA125 as a significant prognostic factor for overall surviva l (P = 0.012; log lank test). Residual tumor at standard surgery after NAC and a bsolute CA125 level were not significant prognostic factors. Based on the CA125 regression rate, it is possible to stratify TIIIc or M1 ovarian serous adenocarc inoma cases into those with a good prognosis of survival and those with poor pro gnosis. Regression coefficient of CA125 level greater than -0.039 predicts good 3-year survival after subsequent radical surgeries. The associations of the CA125 regression rate with initial response to chemothrapy and prognosis remain unclear. We examined the association between CA125 re gression in neoadjuvant chemotherapy (NAC) and prognosis. Fifty patients with ad vanced ovarian cancer (TNM classification TIIIc or M1) who received initial NAC and did not undergo significant cytoreductive surgery were selected for the retrospective analysis, after excluding clear cell carcinoma and mucinous adenocarci noma putative to be cisplatin-resistant. For each patient, regression coefficie nt was calculated using all the CA125 levels measured from the day of NAC as day 0 until the day of normalization of CA125 level (<35 IU / ml) or the day of stan dard surgery. Responder was defined as a regression coefficient of -0.039 or gr eater (33 cases) and nonresponder as a regression coefficient less than -0.039 (17 cases). The 3-year survival rate for all 50 cases was 59.3%. When stratifi ed by regression coefficient of CA125 levels, the 3-year survival was 70.5% in responders and 43.3% in nonresponders. Univariate analysis identified the regres sion coefficient of CA125 as a significant prognostic factor for overall survival 1 (P = 0.012; log lank test). Residual tumor at standard surgery after NAC and a bsolute CA125 level were not significant prognostic factors. Based on the CA125 regression rate, it is possible to stratify TIIIc or M1 ovarian serous adenocarc inoma cases into those with a good prognosis of survival and those with poor pro gnosis. Regression coefficient of CA125 level greater than -0.039 predicts good 3-year survival after subsequent radical surgeries.