BACKGROUND: Differential diagnosis of inguinal mass must include, especially in female patients, a wide variety of lesions among which our analysis will focus on mesothelial cyst of the round ligament of the uterus. A rare developmental lesion often misdiagnosed as hernias and accidentally detected during surgical exploration of the groin region. CASE REPORT: Of a left inguinal mass causing local discomfort and progressive worsening of local pain. A pre-operative diagnosis of left symptomatic femoral hernia was made and the patient consented to surgical treatment. The surgical exploration of the inguinal and femoral canals revealed a femoral hernia associated to a clear fluid cystic lesion of around 2 cm arising from the round ligament. Histopathology demonstrated a mesothelial cyst of the round ligament CONCLUSIONS: Mesothelial cysts of the round ligament of the uterus must be taken into consideration in the differential diagnosis of groin swelling in female patients and a greater effort is needed in order to reach a preoperative diagnosis and prevent an over treatment.