Increasing annual incidence of primary malignant brain tumors in the elderly Between 1973 and 1985, total age-adjusted cancer incidence in the United States (all races, men and women) rose by 10.7%, with an average annual percentage change of +0.9%. Analysis of reported age-specific incidence of primary malignant brain tumors over the same years demonstrates that incidence rates increased dramatically between 1973/1974 and 1985. In 1985, incidence rates for persons aged 75-79, 80-84, and 85 years of age and over were 187%, 394%, and 501%, respectively, of rates in 1973/1974. Similar increases were found in both men and women, analyzed separately and combined. Average annual percentage changes in primary brain tumor incidence were +7.0%, +20.4%, and +23.4% in these age ranges, respectively. Reported incidence in younger persons varied little over the same period of time. The most common histologic type of primary brain tumor in the elderly was of glial origin, predominantly the glioblastoma multiforme and astrocytoma. These tumors are highly malignant and invariably fatal. Two possible causes may explain the increased incidence in the elderly: the introduction and extensive use of x-ray computed tomography since 1973 and/or a true increase in incidence occurring independently of diagnostic advances. 