Chordomas are rare, slow-growing, primary malignant skeletal neoplasms. Chromosome analysis, telomere reduction and telomere activity, DNA microsatellite, and loss of heterozygosity studies have been performed on chordomas; however, the clonality status (monoclonal versus polyclonal proliferation) is unknown. The primary purpose of this study was to determine whether sacral chordoma is monoclonal or polyclonal in origin with the use of a polymorphic X-linked gene (AR; alias HUMARA) and X-chromosome inactivation studies. DNA was harvested from tumor and corresponding normal tissue from eight women (37-71 years) with chordoma. Clonality was determined using an X chromosome inactivation protocol and a polymorphic human androgen receptor gene (AR) located on the X chromosome. The procedure required a methylation-specific polymerase chain reaction (PCR) and determination of the ratio of active to inactive X chromosomes. Results were informative for seven of the eight women, with two separate X-linked alleles seen for the AR gene in the normal tissue. Expression of AR gene alleles from each of the two X chromosomes was present in the chordoma tumor, indicating a polyclonal proliferation in all seven women. Most solid tumors and skeletal neoplasms are polyclonal in nature. Our study indicates that chordoma is polyclonal in its pattern of proliferation.