A number of reports have suggested an increasing incidence of primary brain tumors, especially malignant astrocytomas, in the elderly population. To investigate this issue, we analyzed the incidence and temporal trends of primary intracranial neoplasms diagnosed in the population of Rochester, Minnesota, over the 40 years between 1950 and 1990. The incidence of symptomatic primary brain tumors (excluding patients diagnosed incidentally at autopsy and by neuroimaging studies) increased from 9.5 per 100,000 population per year in 1950 to 1969 to 12.5 per 100,000 per year in 1970 to 1989; this change was not statistically significant (chi 2 trend, 1.89; p = 0.17). While the incidence of pituitary adenomas increased significantly between the two periods (chi 2 trend, 4.44; p = 0.04), the incidence trends of all gliomas, malignant astrocytomas, and meningiomas showed no change among persons younger than 65 years as well as those 65 years and older. The number of patients incidentally found to have neoplasms by neuroimaging studies increased in the recent 20-year period (chi 2 trend, 4.08; p = 0.04). The average age- and sex-adjusted incidence rates per 100,000 per year during the study period in the population of Rochester, Minnesota, for symptomatic tumors were 5.0 for all gliomas, 3.3 for malignant astrocytomas, 2.0 for meningiomas, and 2.4 for pituitary adenomas. In conclusion, our data indicate that the reported increase in the incidence of primary brain tumors is an artifact of improvement in diagnostic technology and practice.