Meningiomas are benign brain tumors thought to arise by multi-step tumorigenesis, involving both the activation of oncogenes and the loss of tumor suppressor genes. The cell cycle regulator proto-oncogene cyclin D1 has been implicated in the pathogenesis of several types of cancer. Northern blot analysis revealed expression of cyclin D1 mRNA in 8 (53%), and cyclin B mRNA in 12 of 14 (86%), primary meningiomas. Immunocytochemistry using an antibody specific for cyclin D1 showed strong positivity amongst meningeal cells in the same meningioma samples. No cyclin D1 mRNA was detected in a sample of normal pachymeninges. Cyclin B, which has not yet been linked to tumorigenesis and serves as a marker for cellular proliferation, was expressed in a higher proportion of meningioma samples. These data provide the first evidence for the overexpression of cyclin D1 and B mRNA and protein in primary human meningiomas, and are consistent with a proposed oncogenic role of cyclin D1 in tumorigenesis. Excessive levels of the cyclin D1 proto-oncogene may lead to deregulation of G1 control in a proportion of arachnoid cap cells leading to tumorigenesis.