Study ObjectiveTo evaluate the efficacy of hysteroscopy in resecting submucous myomas with deep intramural invasion. DesignProspective, observational study (Canadian Task Force classification II–2). SettingDepartment of gynecology at a general hospital. PatientsSixteen women with a solitary submucous myoma, in which myometrial thickness between the outer edge of the myoma and inner edge of the serosa was between 5 and 10 mm. InterventionOne-step hysteroscopic myomectomy. Measurements and Main ResultsMedian myoma diameter and weight were 3.3 cm and 30 g, respectively. Myometrial thickness between the myoma and serosa increased gradually and significantly from 6.7 mm before, to 8.9 mm during, to 16.1 mm immediately after hysteroscopic myomectomy (p <0.001). The thickness of the opposite uterine wall increased from 10.1 mm before, to 11.4 mm during, to 18.8 mm after operation (p <0.001). ConclusionOne-step hysteroscopic myomectomy may be performed to remove deeply infiltrating submucous myomas when myometrial thickness at the implantation site is as thin as 5 mm.