Our objective was to evaluate the kinetic parameters of serum CA125 during neoadjuvant platinum-based chemotherapy (NAC), in patients with epithelial ovarian cancer, in order to identify a surrogate marker of sensitivity to platinum. Patients diagnosed between 2002 and 2009, and treated with NAC and interval debulking surgery, were included in the study. One hundred and forty-two patients met the study inclusion criteria. Fifty-four patients (38%) were platinum-sensitive (PFI >12 months). A CA125 level after the 3rd NAC cycle <35 UI/ml was significantly associated with improved overall survival (OS) and relapse-free survival (RFS). In the multivariate model, patients with a CA125 level after the 3rd NAC cycle >35 UI/ml were 3.8-times more at risk for PFI <12 months (95% CI=1.7-8.5, p<0.001). A CA125 level after the 3rd NAC <35 UI/ml is an independent predictor for tumor platinum-sensitivity.