A 67-year-old woman presented with a rapidly enlarging right anterior thigh
mars. Clinical impression uas of a primary sarcoma. but the histologic ana
lysis of an incisional biopsy specimen showed adenocarcinoma with clear cel
l differentiation. Initially thought to be a metastasis, the malignant tumo
r appears to have arisen from extraabdominal endometriosis of the right gro
in. This case shows clinical, radiographic, and histologic findings that ma
g help the orthopaedic surgeon recognize an unusual complication of extrape
lvic endometriosis.