The purpose of this study was to evaluate the value of CT and MRI in aggressive angiomyxoma (AAM) of the pelvis.A series of four cases from three institutions are reviewed. Among the four cases, three were initially misdiagnosed, and local recurrence necessitated reoperation or angiographic embolization. The fourth case, with accurate preoperative diagnosis, was followed with no recurrence. CT and MR imaging demonstrated a well-defined mass, which displaced adjacent structures. Attenuation of the mass was less than that of muscle on unenhanced CT, and a swirling or layering internal architecture was found using both enhanced CT and T1-weighted MR imaging. In one patient, a layering internal architecture was seen on unenhanced CT images. MRI demonstrated the relation of the tumor to the pelvic floor better than CT. The authors concluded that both CT and MRI show the characteristic imaging pattern and trans-diaphragmatic extent of these tumors, and the diagnosis should be considered in any young woman presenting with a well-defined mass arising from the pelvis or perineum.