AIMS: To determine whether fibromas of tendon sheath represent a reactive fibrosing process or whether they are neoplastic, we investigated the chromosomes of the proliferating cells. A clonally occurring karyotypic abnormality would be an argument in favour of these fibromas being neoplastic. METHODS AND RESULTS: A biopsy specimen of a fibroma of tendon sheath was cultured after collagenase exposure leading to dissociation of the tissue. The cultured cells were harvested after 4 days and chromosome analysis done according to standard procedures. Ten out of 20 karyotyped cells showed an identical chromosome abnormality characterized by a t(2;11)(q31-32;q12). The other 10 cells showed a normal 46,XX karyotype. CONCLUSION: The clonally occurring chromosome abnormality found in this fibroma of tendon sheath suggests that this proliferation is neoplastic and not a reactive fibrosing process.