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Brain tumor incidence has increased over the last 20 years in all age groups, both overall and for specific histologies. Reasons attributed to these increases include increase in lymphoma due to HIV/AIDS, introduction of computed tomography/magnetic resonance imaging, and changes in coding/classification. The purpose of this study was to describe overall and histologic-specific incidence trends in a population-based series of primary benign and malignant brain tumors. Data from the Central Brain Tumor Registry of the United States from 1985 through 1994 were used to determine incidence trends in the broad age groups 0-19, 20-64, and > or = 65 years, both overall and for selected histologies. Poisson regression was used to express trends as average annual percentage change. Overall, incidence increased modestly (annual percentage change 0.9%, 95% confidence interval, 0.4, 1.4). When lymphomas were excluded, this result was not statistically significant (annual percentage change 0.5%, 95% confidence interval, -0.1, 1.1). Specific histologies that were increasing were lymphomas in individuals aged 20 to 64 years and in males aged 65 years or older, ependymomas in the population aged 20 to 64 years, nerve sheath tumors in males, and pituitary tumors in females. Increases that were not specific to any population subgroup were seen for glioblastoma, oligodendrogliomas, and astrocytomas, excluding not otherwise specified (NOS) tumors. Corresponding decreases were noted for NOS, astrocytoma NOS, and glioma NOS. Increasing incidence trends for lymphomas were consistent with previous literature. Improvements in diagnostic technology in addition to changes in classification and coding were likely to be responsible for decreases seen in incidence of NOS subgroups and corresponding increases in glioma subgroups. In contrast, the increases identified for ependymomas, nerve sheath tumors, and pituitary tumors were less likely to be artifacts of improvements in diagnosis, and they warrant further study.