Six cases of Stage I adenosarcoma of the uterus are reported. Although these neoplasms have generally been regarded as being of low malignant potential with little propensity for distant metastasis, our experience reveals some of them to be aggressive tumors with rapid clinical demise. Five (83%) patients demonstrated recurrence after initial surgery, three despite postoperative vaginal radium or external pelvic radiation. Four of the recurrences occurred in the pelvis and abdomen; the fifth was limited to the vagina. Three patients were dead of disease within three years of diagnosis, two of whom achieved clinical remission for at least one year to combination chemotherapy. One of the remaining three patients died from intercurrent disease without recurrence, and two are alive without disease after treatment for recurrence. Based on this limited experience, we believe these neoplasms should be regarded as potentially as malignant as their mixed Müllerian counterpart.