Osteosarcomatous transformation in fibrous dysplasia is unusual. The incidence is increased in patients with concomitant Mazabraud's syndrome and McCune-Albright syndrome. We report the cytological, histological, and cytogenetic findings of this rare entity arising from a mass in the right elbow of a 44-year-old African-American woman. The fine-needle aspiration (FNA) findings were diagnostic of malignancy, with markedly atypical spindle and polygonal cells admixed with osteoid. The diagnosis of osteosarcoma by FNA was subsequently further confirmed by histological evaluation of an above-elbow amputation specimen. Fluorescence in situ hybridization and comparative genomic hybridization demonstrated trisomies of chromosomes 5 and 7 in the fibrous dysplasia and osteosarcoma. In addition, multiple chromosomal abnormalities were also noted in the osteosarcoma. We are unaware of any previous reports of the cytogenetic findings in the tissue of this rare condition, and argue for the value of FNA in the evaluation of such patients under selected conditions. © 2003 Elsevier Inc. All rights reserved.