Biclonal chromosome complements in uterine leiomyoma have been reported occasionally. These previous studies reported the presence of two unrelated clones containing mainly t(12;14) and del(7). We describe four cases of typical leiomyoma displaying two clones, both involving chromosome 7 but with a different deletion in each of the two clones. For two of the tumors, the biclonal origin is the only possible explanation; for the remaining two cases, the origin of the two deleted chromosomes 7 could also be explained by clonal evolution, since the more proximal deletion on chromosome 7 in one clone appears to be subsequent to the deletion of the other clone. Even in these cases, however, the biclonal origin cannot be excluded completely. Despite the mechanism of origin, deletion of chromosome 7 is the most common cytogenetic abnormality in leiomyoma, indicating that loss of genetic material from the long arm of this chromosome is critical for tumor development.