Malignancy of myomas is rare, with an incidence of about 0.4% to 0.8%. Malignancy of smooth muscle sarcoma of the uterus occurs mostly in older women and is rare in women under 30 years of age. Malignant transformation should be alerted to the following points: rapid growth of fibroids in the short term; fibroids with irregular bleeding; fibroids continuing to increase in postmenopausal women; and ultrasound suggesting a very rich blood supply to the fibroids. Malignancy of fibroids mostly occurs in larger interstitial fibroids, followed by cervical fibroids.