In 2018, pembrolizumab, an anti-PD-1 antibody, was approved for treatment of microsatellite instability (MSI)-high solid tumors in Japan. Here, we retrospectively evaluated the relationship of patient background, treatment efficacy, adverse events, and treatment course with the neutrophil-to-lymphocyte ratio (NLR) in peripheral blood of patients treated with pembrolizumab for MSI-high gynecological cancer. Among 128 patients with recurrent gynecological cancer, the MSI positivity rate was 9.4% (12 patients) of which eight patients (median age, 71 years) were treated with pembrolizumab. The response rate was 50% (4/8 patients). The median progression-free period was 10 months, and the median duration of response in the responders was 12.5 months. Adverse events included arthralgia, edema, decreased vision, and pruritus, all of which were grade 1. Of the eight patients who received pembrolizumab, one underwent craniotomy for a benign brain tumor, and another, palliative irradiation for a recurrent pelvic lymph node. In addition, in six of the eight cases, the peripheral blood NLR fluctuated in conjunction with the treatment effect and tumor markers. These results confirm the clinical efficacy and safety of pembrolizumab against MSI-high gynecological cancer. They also support estimation of the treatment effect by evaluating change in the NLR. [Adv Obstet Gynecol, 75(1) : 10-15, 2023 (R5.2)]