CD138 (syndecan-1) expression in bone-forming tumors. CD138 (syndecan-1), a cell surface proteoglycan, is sensitive and specific for plasmacytic differentiation in hematologic disorders. Expression of CD138 has been observed in a majority of epithelial neoplasms and, rarely, soft tissue tumors. However, its expression in bone tumors has not been evaluated. We studied CD138 expression in 27 osteosarcomas, 12 benign bone-forming tumors (osteoid osteoma and osteoblastoma), and 17 reactive bone cases. CD138 expression was also evaluated in a tissue microarray (TMA) constructed from 24 osteosarcomas, 24 chondrosarcomas, 12 giant cell tumors of bone, and 9 normal bone samples. Membranous expression of CD138 was found in an average of 31% of osteosarcoma cases (16/51; 14/27 [52%] in in-house cases; 2/24 [8%] in TMA cases) and in 83% of osteoid osteoma/osteoblastoma cases (10/12). Subsequent immunoglobulin κ and λ stains were negative in the CD138+ cases. All cases of chondrosarcoma, giant cell tumor of bone, and normal/reactive bone tested were nonreactive with anti-CD138. Our results show that CD138 reactivity for neoplastic cells in bone is not a definitive marker for plasmacytic origin, and caution is required to interpret CD138+ cells from a bony lesion for which a hematologic etiology has not been established.