Objectives: A subset of patients with high-grade serous ovarian cancer (HGSOC) treated with neoadjuvant chemotherapy (NACT) has a disease that is resistant or refractory to treatment. The objective of this study was to externally validate the KELIM score in this patient cohort compared to patients with the platinum-sensitive disease and to determine whether the KELIM score is predictive of platinum sensitivity, progression-free (PFS), and overall survival (OS). Methods: This is a retrospective cohort study of NACT patients with Stage III-IV HGSOC diagnosed between January 1, 2010, and December 31, 2019, at a tertiary cancer center. KELIM score was calculated using at least three CA-125 values within the first 100 days of chemotherapy using a validated calculation formula [https://www.biomarker-kinetics.org/CA-125-neo], notably pre-cycle 2, 3 and 4. Demographic parameters were collected, and Kaplan-Meier survival analyses were performed for PFS and OS. This study was approved by the local ethics board. Conclusions: Patients with advanced HGSOC undergoing NACT with a calculated KELIM score <1 were more likely to have a platinum-resistant disease, worse PFS, and worse OS when compared to patients with a KELIM score ≥1. The KELIM score can be a helpful tool to predict chemotherapy sensitivity and survival outcomes in this patient population and may aid in treatment decision-making. Objectives: A subset of patients with high-grade serous ovarian cancer (HGSOC) treated with neoadjuvant chemotherapy (NACT) has a disease that is resistant or refractory to treatment. The objective of this study was to externally validate the KELIM score in this patient cohort compared to patients with the platinum-sensitive disease and to determine whether the KELIM score is predictive of platinum sensitivity, progression-free (PFS), and overall survival (OS). Methods: This is a retrospective cohort study of NACT patients with Stage III-IV HGSOC diagnosed between January 1, 2010, and December 31, 2019, at a tertiary cancer center. KELIM score was calculated using at least three CA-125 values within the first 100 days of chemotherapy using a validated calculation formula [https://www.biomarker-kinetics.org/CA-125-neo], notably pre-cycle 2, 3 and 4. Demographic parameters were collected, and Kaplan-Meier survival analyses were performed for PFS and OS. This study was approved by the local ethics board. Conclusions: Patients with advanced HGSOC undergoing NACT with a calculated KELIM score <1 were more likely to have a platinum-resistant disease, worse PFS, and worse OS when compared to patients with a KELIM score ≥1. The KELIM score can be a helpful tool to predict chemotherapy sensitivity and survival outcomes in this patient population and may aid in treatment decision-making.