Case Report: A 39-year-old woman, para 0, complained of a painful mass in her right inguen.Ultrasonography (US) showed a hypoechoic mass measuring 20×13 mm across in the inguinal muscle coat. The examination showed the internal echo to be comparatively homogenous and hypoechoic but mixed with a minute strong echo, and there was posterior enhancement. A clear boundary and good mobility were also observed. The shape could not be changed by dynamic test and resembled that of a tadpole with its tail extended toward the abdominal cavity. Power Doppler imaging (PWD) and pulsed Doppler analysis showed a relatively large number of snaking colored signals as well as high pulsatility flow indices (PI) in the mass. After surgery the patient recalled increased tenderness and perhaps swelling of the mass during menstrual periods. Discussion: We presumed that the image of the mass would have a characteristic shape and that the internal echo would exhibit the same changes as those observed in the normal endometrium during the menstrual cycle. In this case, the mass was composed mainly of endometrial tissue, which is different from the hemorrhagic blood in ovarian endometriosis. However, we were unable to demonstrate any changes, and indeed such a demonstration may not be possible, that could be proven to be characteristic of endometriosis. Conclusion: US can support the clinical diagnosis of endmetriosis in an hernia sac by establishing the characteristic shapes, and it can also be used frequently in following-up examination.