There is a prevailing view that leiomyomas of deep soft tissue are rare or nonexistent, but there are limited data on this subject in the form of large clinical studies with long follow-up information. We reviewed 36 consultation cases that had been diagnosed as leiomyoma or probable leiomyoma based on absence of nuclear atypia, necrosis, and no/minimal mitotic activity. Follow-up information was obtained to determine whether these stringent histologic criteria could identify a biologically benign group of smooth muscle tumors of deep soft tissue. The tumors occurred in two distinct locations. The first (n = 13) occurred in deep somatic soft tissue of the lower extremity (7), upper extremity (2), trunk (2), axilla (1), and back (1) and affected the sexes equally (7 male, 6 female). Composed of a circumscribed mass of mature smooth muscle cells, they were frequently calcified with a mean mitotic activity of <1 mitosis/50 high power fields (HPF) (range 1-4 mitoses/50 HPF). Estrogen receptor and progesterone receptor proteins were negative in the three cases tested. No tumors recurred or metastasized (mean follow-up 58.7 months, range 5-97 months). The second group (n = 23) occurred within the retroperitoneum (20) or abdominal cavity (3) of women (1 male, 22 female). Resembling uterine leiomyomas, they were always distinct from the uterus, occasionally multiple (n = 4), and sometimes occurred up to years after hysterectomy (n = 3). Four cases occurred with synchronous uterine leiomyomas. In the six cases tested, five of six were positive for the estrogen receptor protein and all were positive for progesterone receptor protein. Mean mitotic activity was 1 mitosis/50 HPF (range <1-10 mitoses/50 HPF). None developed metastasis within the follow-up period (mean 42.5 months, range 6-120 months); one tumor with a positive margin recurred at 10 months. We conclude that clinically benign smooth muscle tumors of deep soft tissue are rare but can be identified using stringent histologic criteria. They comprise two distinct subtypes: leiomyomas of somatic soft tissue and retroperitoneal-abdominal leiomyomas. The latter probably arise from hormonally sensitive smooth muscle. Although similar to uterine leiomyomas, they are located at sites removed from the uterus and are likely independent soft tissue primaries rather than parasitic leiomyomas of the uterus. We suggest that these two groups of smooth muscle tumors be diagnostically approached in a site-specific fashion.