Regulatory T cells (Treg) suppress immune responses in patients with cancer. Surgery is the most effective therapeutic strategy for ovarian cancer (OC). However, the interplay between the Treg population and surgical resection remains unclear. 61 patients with OC who received no prior treatment were enrolled in the study. Treg percentages were characterized from peripheral blood mononuclear cells. We investigated CD4+ CD25+, CD4+ CD25+ Foxp3+, CD8+ CD28−, and CD8+ Foxp3+ Tregs in OC patients and their postoperative changes using flow cytometry. Treg percentages were significantly higher in OC patients than those in benign ovarian tumors (BOT) and healthy controls. Higher percentages of Tregs were found in patients with stage III/IV than stage I/II OC. Treg percentages were significantly decreased postoperatively. The postoperative Treg percentages in patients with stage I/II OC were similar to those in BOT patients, while postoperative Treg percentages in patients with stage III/IV OC remained higher. Tregs were markedly lower on postoperative day (POD) 3 than preoperatively. They increased slightly after 7 days, but remained lower than preoperative levels. These data suggested that Tregs continued to decline from POD 7 to POD 30. Treg percentages are correlated with the tumor burden and could be a key factor in monitoring the immunological status of patients with OC.