Platelet-derived growth factor (PDGF) is a major mitogen and chemotactic factor for mesenchymal cells such as fibroblasts, smooth muscle cells, and osteoblasts. PDGF exists as disulfide-linked homo- or heterodimers composed of two polypeptide chains encoded by distinct genes, designated PDGF-A and PDGF-B. Upon binding to its tyrosine kinase receptor PDGF-alpha, especially PDGF-AA stimulates the proliferation of osteoblastic cells and may exert autocrine and paracrine effects in regulating bone-forming processes. The purpose of this immunohistochemical study was to determine the expression of PDGF-AA and PDGF-alpha receptor in benign and malignant neoplastic bone lesions. Polyclonal antibodies to PDGF-AA and PDGF-alpha receptor were used on paraffin sections of 23 osteosarcomas and 17 osteoblastomas. Immunostaining was assessed quantitatively by evaluating the percentage of reactive tumor cells. In osteosarcomas, the mean expression of PDGF-AA and PDGF-alpha receptor was 33.97% (range, 2 to 80%; SD, 24.26%) and 27.13% (range, 3.2 to 72%; SD, 18.38%), respectively. Osteoblastomas showed significantly lower expression of PDGF-AA than osteosarcomas (mean, 15.71%; range, 5 to 34%; SD, 9.43%; P = .019). Although the mean expression of PDGF-alpha receptor in osteoblastomas was much lower than in osteosarcomas (mean, 17.55%; range, 3.6 to 26.8%; SD, 6.47%), the difference was not significant (P = .122). For osteosarcomas, Spearman correlation coefficient (two-tailed) revealed a significant correlation between the expression of PDGF-AA and PDGF-alpha receptor (r = .688), which was not the case for osteoblastomas (r = .267). These data suggest that in contrast to osteoblastoma, the growth of osteosarcoma may be supported by the coordinate expression of the potent mitogenic growth factor and its receptor that exert their functions by autocrine and paracrine mechanisms.