Five adamantinomas of long bones were cytogenetically characterized to investigate the role of chromosomal aberrations in their histogenesis, as well as a putative relationship between adamantinoma and osteofibrous dysplasia (OFD). Three tumors had a classic histologic subtype, with abundant epithelium. Two of them revealed trisomies 7, 8, 12, and 19, combined with a balanced translocation, t(10;12), with centromere breakpoints in one tumor. The third showed a karyotype 51,XY, +X, +7, +12, +19, +21. The fourth tumor, of OFD-like subtype, showed trisomies 7, 8, and a small marker chromosome in a low percentage of cells. The fifth tumor, also of OFD-like subtype, displayed only a few keratin-positive cells from the multiple tissue blocks investigated. This latter tumor revealed a clonal abnormality with a karyotype 46,XX,t(2;11)(p23;q14)inv(11)(p14q14), which was confirmed with fluorescence in situ hybridization (FISH), using chromosome-specific library probes and chromosome 11 locus-specific probes. The trisomies 7, 8, and 12 also were described in OFD, which suggests a common histogenesis of OFD and adamantinoma. Our findings further support the probability of clonal origin of OFD. The OFD-like component may be an integral element of adamantinoma, rather than a tissue reaction to epithelial tumor cells.