Purpose of investigation: To assess the ability of detecting the number of uterine myomas by transvaginal ultrasonography (TVS) performed supporting the clinical examination of general gynecologists’ office practice. Methods: A retrospective comparison of the number of myomas revealed by preoperative in-office TVS and documented after laparotomic myomectomy was conducted in 110 consecutive premenopausal patients referred for surgery. Results: The sensitivity of TVS in revealing the exact number of myomas was 59.4% in the whole series. In the subgroup of 88 patients with a preoperative diagnosis of three or fewer myomas TVS missed at least one myoma in 31 (35.2%) cases, achieving a 64.8% sensitivity. Among the 72 women diagnosed with one myoma at preoperative TVS, 19 (26.4%) resulted to have two or more myomas at the end of surgery, reaching a 73.6% sensitivity of TVS in revealing the exact number of myomas. Conclusions: In-office TVS reinforces the clinical diagnosis of uterine myomas but it often fails in the detection of their number, resulting in a poor preoperative characterization of patients. The fact that one myoma may be overlooked in one-third of patients theoretically eligible for laparoscopic conservative surgery may motivate the implementation of US diagnosis when laparoscopic myomectomy is considered.