Introduction: Experimental studies suggest that traffic-related ultrafine particles can cross blood-brain barrier leading to neurotoxicity and development of brain tumors in animals, but epidemiological evidence in humans is sparse. We examine association between long-term exposure to air pollution and incidence of brain cancer, for the first time with particulate matter with diameter < 2.5 (PM2.5). Methods: For 28,731 nurses (age ≥ 44 years) from the Danish Nurse Cohort, recruited in 1993 or 1999, we obtained data on brain cancer incidence until 2013 from Danish Cancer Register, and estimated levels of PM2.5, particulate matter with diameter <10 µm (PM10), nitrogen oxides (NOx) and nitrogen dioxide (NO2) at residence since 1990 using a dispersion model. We examined association between 3-year running mean of pollutants and brain tumor incidence using a time-varying Cox regression, separately for total brain cancer, and for subtypes by location (brain or meninges) and malignancy (malignant or benign). Results: Of 25,143 cancer-free nurses at recruitment 121 developed brain tumor during 15.7 years of follow-up. We found weak positive association between total brain cancer and PM2.5 (Hazard Ratio: 1.06; 95% Confidence Interval: 0.80-1.40) per interquartile range (IQR, 3.37 µg/m3) and NO2 (1.09; 0.91-1.29) per 7.5 µg/m3 and none with NOx or PM10. Associations with PM2.5 and NO2 were stronger for tumor located in meninges than in brain, and for benign than for malignant tumors. Finally, associations of total brain cancer with PM2.5 were modified by body mass index and were statistically significantly enhanced in obese women (2.03; 1.35-3.05). Conclusion: We found weak evidence of association between long-term exposure to PM2.5 and development of brain cancer in women older than 44 years. Findings of relevance of air pollution for meninges and benign tumors, as well that obese women may be most susceptible, requires further investigation.