Cytogenetic analysis was performed on short-term cultured tumor cells from ten patients diagnosed as having leiomyosarcoma. Of these, five tumors from five unrelated patients had clonal chromosome abnormalities. The combined data from this report and previous studies on leiomyosarcomas indicate that at least three subtypes may be identified chromosomally within leiomyosarcomas. One subtype is characterized by a hypodiploid chromosome number ranging from 41 to 43 and a common chromosome pattern of monosomies of chromosomes specific to this subgroup, including partial monosomy of the short arm of chromosome 1 (1p13----pter), monosomy 18, and consistent monosomy 22. In contrast to the previous subtype, for the pathogenesis of which the "tumor suppressor gene" hypothesis may be suggested, another subtype was characterized by a pseudodiploid chromosome number associated with simple reciprocal translocations; so far, these translocations are unique to individual tumors, the pathogenesis of which may involve a translocation-mediated gene deregulation pathway. A third subtype contained tumors with heterogeneous karyotypic findings.