The author reviews the molecular genetics, pathology, and cell kinetics of meningiomas and the role that regional multiplicity in the dura mater may play in their recurrence. Malignant and radiation-induced meningiomas are discussed, with summaries of series of 60 patients with frankly malignant lesions treated over a period of 22 years at the University of California, San Francisco, and of 10 patients with meningiomas induced by high-dose radiation therapy. Reviewing a 23-year series of 140 patients with subtotally removed meningiomas who were treated post-operatively with radiation, the author recommends that, with meticulous technique, irradiation is effective in preventing the regrowth of subtotally removed benign meningiomas and of all malignant meningiomas. Adoption of both the microscopical cytological grading system proposed by Jääskeläinen's group in Helsinki and the classification of operations proposed by Donald Simpson is also recommended. Wide removal of dura adjacent to meningioma reduces the risk of recurrence, and determination of the bromodeoxyuridine labeling index provides a valid basis for planning treatment and follow-up evaluations. Increased awareness is necessary for early recognition of radiation-induced meningiomas in patients at risk for developing such tumors. For meningiomas in such sites as the parasellar region and the posterior fossa, conservative removal of tumor followed by irradiation is advocated in preference to a radical operation that may cause neurological injury without being curative.