Objective: In patients with advanced stage epithelial ovarian cancer (EOC) the volume of residual tumor after debulking is known as prognostic factor for survival. We wanted to examine the relationship between postoperative decline in serum CA125 and residual disease after cytoreductive surgery and evaluate perioperative changes in serum CA125 levels as predictor for disease-specific survival. Methods: A retrospective study was conducted of patients with FIGO stage IIb-IV EOC treated with cytoreductive surgery, followed by chemotherapy between 1996 and 2010 in three hospitals in the Southeastern region of the Netherlands. Data were analyzed with the use of multilevel linear regression and Cox-proportional hazard regres sion models. Results: A postoperative decline in serum CA125 level of ≥80% was associated with complete primary cytoreduction (p=0.035). Univariate analyses showed favorable associations with survival for both the degree of decline in serum CA125 and residual tumor after primary cytoreduction. In multivariate analyses the decline in serum CA125 but not the outcome of surgery remained significantly associated with better survival (HR50%-79%=0.52 [95% CI: 0.28-0.96] and HR≥80%=0.26 [95% CI: 0.13-0.54] vs. the serum CA125 decline of < 50% [p < 0.001]). Conclusion: The current study, although hampered by possible biases, suggests that the perioperative decline in serum CA125 is an early biomarker that predicts disease-specific survival in patients who underwent primary cytoreductive surgery for advanced stage EOC. If confirmed prospectively, the perioperative change in serum CA125 could be a better marker for residual tumor volume after primary cytoreductive surgery (and therewith disease-specific survival) than the surgeons' estimation of residual tumor volume.