OBJECTIVE: This study was performed to quantify hidden blood loss (HBL) and explore its influential factors in myomectomy. METHODS: Two hundred nine patients who underwent myomectomy by laparotomy or laparoscopy from 1 January 2017 to 31 December 2018 were analyzed. Each patient's estimated blood volume and total blood loss (TBL) were calculated by the Nadler formula and Gross formula, respectively. The HBL was calculated by subtracting the visible blood loss (VBL) from the TBL. A multivariate linear stepwise analysis was applied to identify the influential factors of HBL in myomectomy. RESULTS: The mean perioperative VBL and estimated TBL during myomectomy were 137.81 ±104.43 and 492.24 ± 225.00 mL, respectively. The mean HBL was 354.39 ± 177.69 mL, which accounted for 71.52% ± 15.75% of the TBL and was two to three times higher than the VBL. The duration of surgery, number of removed leiomyomas, and location of removed leiomyomas were independent risk factors for HBL in myomectomy. CONCLUSIONS: HBL accounted for a significant percentage of TBL in myomectomy. A full understanding of the HBL in perioperative blood management may improve patients' postoperative rehabilitation.