A leiomyoma (plural is 'leiomyomata') is a benign smooth muscle neoplasm that is not premalignant. They can occur in any organ, but the most common forms occur in the uterus and the esophagus.
Uterine fibroids are the most common neoplasm in females, with 25% of all caucasian and 50% of all black reproductive females having it.
Leiomyomas are estrogen-responsive, which causes rapid growth during pregnancy in some, and for them to often regress or shrink after menopause or removal of the ovaries. Usually, patients are asymptomatic. Important symptoms include abnormal genital tract bleeding, pain, infertility, urinary frequency, abortion, pregnancy-related bleeding, or interference with the position of the fetus.
In terms of its pathology, leiomyomas grossly appear as round, well circumscribed, solid nodules that are white, or tan whorled. There are usually multiple leiomyomas in three sites - intramurally (most common), submucosal, and subserosal. Other changes that may be detected grossly are hemorrhage, necrosis, or cystic changes. Less frequently, leiomyomas may occur at the lower uterine segment, cervix, or uterine ligaments.
Microscopically, bundles of smooth muscle cells can be seen. These cells are uniform in size and shape, with scarce mitoses. There are three benign variants: bizarre (atypical); cellular; and mitotically active.
Diagnosis is done by endometrial sampling if the leiomyoma is submucosal, and in resected samples, such as those from a myomectomy, or hysterectomy.
They are also the most common benign esophageal tumour, though this accounts for less than 1% of esophageal neoplasms  (http://www.emedicine.com/med/topic738.htm). As such, the discussion below is on gynecologic leiomyomas.