We observed clonal chromosome abnormalities in two fibrous soft tissue tumors diagnosed as collagenous fibroma (desmoplastic fibroblastoma). The involvement of the same band of the long arm of chromosome 11, 11q12, was observed in both tumors. The presence of hitherto unreported similar chromosomal abnormalities in this tumor supports the neoplastic nature of this lesion. In addition, a possible relationship with fibroma of tendon sheath, which also shows rearrangement of 11q12, is suggested. 11q12 might be a common genetic denominator of benign fibroblastic lesions, such as collagenous fibroma and fibroma of tendon sheath.