Objective: To evaluate and compare hidden blood loss (HBL) in laparoendoscopic single-site myomectomy (LESS-M) and conventional laparoscopic myomectomy (CLM). Design: Retrospective cohort study. Setting and population: Patients underwent laparoscopic myomectomy from July 2019 to October 2020 at the Second Affiliated Hospital of Wenzhou Medical University Methods: We collected data and calculated the HBL according to the Nadler and Gross formula, and identified its risk factors by multiple linear regression analysis. Main outcome measures: Visible blood loss(VBL)，total blood loss(TBL), and HBL. Results: The VBL was 115.4±180.6 mL in the LESS-M group; however, the HBL was 364.3±252.6 mL, accounting for 74.4±22.4% of true TBL. The VBL in the CLM group was 187.9±198.5 mL, and the HBL was 306.8±304.7 mL, accounting for 58.9±30.2% of true TBL. The HBL as a % of true TBL in LESS-M group was more than CLM group (p=0.000). Conclusions: HBL accounted for a significant percentage of TBL in conventional laparoscopic myomectomy. Paying attention to perioperative blood changes and a full understanding of the HBL might promote postoperative recovery of patients. Funding：This study was financially supported by the Medical and Health Research Project of Zhejiang Province of China [grant No. 2018KY127], the Natural Science Foundation of Zhejiang Province [grant No. LY20H040005], the Public Welfare Science and Technology Project of Wenzhou City [grant No. Y2020087]. Keywords: Hidden blood loss, laparoendoscopic single-site myomectomy, conventional laparoscopic myomectomy, leiomyoma, risk factors.