Benign giant cells resembling osteoclasts are seen in about 25% of osteosarcomas. In rare osteosarcomas, benign giant cells are so numerous that they may obscure the malignant cells in the background and lead to the mistaken diagnosis of a giant cell tumor. This is more likely to occur in sacrum - a frequent location for giant cell tumor. The radiographic differences between the giant cell tumor and osteosarcoma are less apparent in the sacrum than in the long bones. It is worth remembering that usual giant cell tumor occurs in skeletally mature patients. A tumor that appears to be a giant cell tumor but arising in a skeletally immature person should be sampled thoroughly and carefully to exclude osteosarcoma rich in giant cells.