Uterine fibroids (leiomyoma) are usually benign tumors that are in the smooth muscle tissue of the uterus (womb). Overall, these tumors are fairly common and occur in about 50% to 80% of women by the time they reach age 50. Uterine fibroids can be as small as a few millimeters (less than an inch) in diameter. They can also be very large (grapefruit-size or larger).