Two cases of solitary plasmacytomas of the skull are presented, and some biological aspects of the tumor examined. A 75-year-old woman presented with a tumor in the right parietal region. The serum level of immunoglobulin G (IgG) was high and a urine test for Bence Jones protein was negative. A reddish vascular mass was totally removed at surgery. The serum level of IgG was within normal limits after the operation. Postoperative radiotherapy was not performed. A 58-year-old woman presented with a tumor in the occipital region. Serum levels of Igs were within normal limits. A urine test for Bence Jones protein was positive for Ig kappa chain. Bone marrow aspiration revealed no evidence of systemic myelomatosis. The tumor mass was totally removed at surgery and she received local radiation therapy (total 50 Gy). Three months after the surgery, Bence Jones protein (kappa chain) was detected in both the urine and serum and bone scintigraphy showed a weak hot spot in the iliac bone, suggesting development to multiple myeloma. Immunohistochemical studies showed that most tumor cells were positive for vascular endothelial growth factor and syndecan-1, and some tumor cells were strongly positive for basic fibroblast growth factor in both cases. The Ki-67 staining indices were 11.3% and 15.6%. Tumor tissues were negative for p53. These results suggest that solitary plasmacytoma of the skull expresses the angiogenic factors, vascular endothelial growth factor, and basic fibroblast growth factor, in accordance with the high vascularity of the tumors, and syndecan-1 may be an immunohistochemical marker of solitary plasmacytoma of the skull.