The frequency of meningiomas has been the topic of relatively few reports. Hospital-based brain tumor series indicate that the incidence is approximately 20% of all intracranial tumors; population-based studies indicate an overall incidence of 2.3/100,000. Although intracranial tumors as a whole show a higher prevalence in males than in females, meningiomas have a 2:1 female-to-male ratio. Between Caucasians and Africans, African-Americans, and Asians, certain differences also have been noted. Meningiomas in children are rare and differ from those in adults and other childhood tumors; they are even more rare in infants. Several features indicating etiologic factors have been identified, among which are ionizing radiation, head injury, hormones, and other receptor binding sites, genetic factors, and viruses. The most common source of exposure of the head to ionizing radiation is dental radiographic examination. Since 1922, head trauma has been considered a possible risk factor, but recent large studies do not support this link. Several factors have prompted studies of estrogens and progestogens as risk factors for meningiomas. Other studies have sought to determine if certain individuals have an inherited predisposition for developing a meningioma and/or if viruses, which may act alone or with other mutagens, figure into the formation of a meningioma. The most promising studies are those of cytogenetics, and future elucidation of factors associated with the loss of one copy of chromosome 22, another phenomenon that has been identified in meningiomas, may lead to screening tests and gene therapy.