Aggressive angiomyxoma is a distinctive soft tissue tumor associated with a high risk of local recurrence but lacks metastatic potential. This tumour occurs nearly exclusively in the soft tissues of the pelvis and perineum of adult women. The line of differentiation is not firmly established, but a fibroblastic/myofibroblastic origin has been proposed. We report 16 new cases of aggressive angiomyxoma of the pelvic soft tissue in women. In all cases bundles of cells, most often adjacent to vessels, with histological features of smooth muscle cells were identified. In 11 of 14 cases the myoid bundles were immunoreactive for desmin; they were also positive for smooth muscle actin in 10 of 11 cases. In 13 of 14 cases lesional stromal cells showed immunoreactivity for desmin. Three cases showed areas with histological features similar to those of angiomyofibroblastoma of the vulva, thus representing previously undescribed morphological overlap between these two entities. We conclude that aggressive angiomyxoma and angiomyofibroblastoma are related neoplasms in a spectrum of tumours showing myofibroblastic origin. Furthermore, the demonstration of immunoreactivity for desmin in aggressive angiomyxomas implies that this antibody is not helpful in discriminating between these two tumours, and the principal means of distinction remains histomorphological analysis.