BACKGROUND: Primary benign and borderline (BB) brain tumors have been reportable since 2004 by population-based cancer registries in the United States. Because these tumors often are diagnosed clinically at nonhospital settings, underreporting is a big concern. Despite this, the magnitude and geographic variations in underreporting are unknown. The objectives of this study are to assess variations in BB brain tumor incidence rate by registry and trend in comparison to malignant brain tumors, as well as to identify the factors associated with the completeness of BB brain tumor reporting. METHODS: North American Association of Central Cancer Registries (NAACCR) Cancer in North America (CINA) Deluxe 1995–2012 Analytic File, which included data from 47 US population-based cancer registries, was used. Age-adjusted incidence rate and average annual percent change (APC) were calculated. Correlation coefficients were used to assess the relationships between incidence rates and clinical factors. RESULTS: The overall age-adjusted incidence rate was 14.2 per 100,000 for BB brain tumors and 6.6 per 100,000 for malignant brain tumors. The age-adjusted incidence rates of BB brain tumors varied by registry from 9.8 per 100,000 to 19.9 per 100,000, whereas the variations in malignant brain tumors were much smaller from 4.1 per 100,000 to 7.7 per 100,000. BB brain tumor cases were more likely than malignant brain tumors to be diagnosed through radiography without microscopic confirmation or surgery. Overall, the BB brain tumor incidence rate significantly increased by 2.3% per year from 2004 to 2012. In contrast, incidence rates of malignant brain tumors significantly decreased by 0.9% per year in the same period. Higher BB brain tumor incidence rates were significantly associated with higher proportions of cases without microscopic confirmation or surgery. These associations were not observed for malignant brain tumors. CONCLUSIONS: Incidence rates of BB brain tumors varied substantially across 47 US registries and were higher than those of malignant brain tumors in the United States. The variations in incidence rate of BB brain tumors may be largely attributable to difference in identifying clinically diagnosed cases. The increasing incidence rate of BB brain tumors may reflect improved case ascertainment rather than a biological trend. Key words: