Purpose <p>Multiple studies have found an inverse relationship between asthma and malignant gliomas, possibly due to increased immune response from asthma. This study assessed the relationship between asthma and subsequent glioma diagnosis while reducing potential effects of recall and detection bias.</p> Methods <p>Data were extracted from the Military Cancer Epidemiology database which includes cancer registry and medical claims data from military and retired servicemembers and their dependents. Cases were individuals with a pathology-confirmed glioma diagnosis between 1998 and 2014. Cancer free-controls were matched to cases by sex, race, age, and active-duty military status. Conditional logistic regression was used to calculate odds ratios (OR) and 95% confidence intervals for the relationship between asthma and subsequent glioma diagnosis.</p> Results <p>Cases were less likely to have been diagnosed with asthma than controls [OR = 0.72 (0.57, 0.92)], specifically among high-grade gliomas [OR = 0.71 (0.52, 0.98)] and large (&gt; 37&#xa0;mm) tumors [OR = 0.60 (0.39, 0.93)]. Cases were less likely to have been diagnosed with asthma 13–36 months [OR = 0.53 (0.33, 0.87)] and &gt; 36 months [OR = 0.75 (0.56, 0.99)] before a glioma diagnosis.</p> Conclusion <p>We found an inverse association between asthma and subsequently diagnosed malignant glioma, notably among high-grade gliomas and large tumors. However, no association was observed for small tumors or with asthma diagnosed within 12 months prior to glioma diagnosis, suggesting that greater surveillance after asthma might increase the detection of a glioma. These findings suggest that potential detection bias might have obscured the inverse association between asthma and glioma, particularly among low-grade gliomas and circumscribed gliomas and glioneuronal tumors.</p>