the objective of this study was to evaluate the bidirectional association between asthma and migraines using control subjects matched by demographic factors. the Korean Health insurance Review and Assessment Service-National Sample Cohort from 2002 to 2013 was used. In study I, 113,059 asthma participants were matched with 113,059 control I participants. In study II, 36,044 migraine participants were matched with 114,176 control II participants. The hazard ratios (HRs) of migraines in the asthma patients (study I) and asthma in the migraine patients (study II) were analyzed using stratified Cox proportional hazard models after adjusting for depression and the Charlson comorbidity index. In study I, 5.3% (6,017/ 113,059) of the asthma group and 3.4% (3,806/ 113,059) of the control I group had migraines (P < 0.001). The asthma group demonstrated an adjusted HR of 1.47 for migraine (95% confidence interval (CI) = 1.41-1.53, P < 0.001). In study II, 15.4% (5,548/36,044) of the migraine group and 10.6% (15,271/144,176) of the control group had asthma (P < 0.001). The migraine group showed an adjusted HR of 1.37 for asthma (95% CI = 1.33-1.41, P value < 0.001). Asthma and migraines are reciprocally associated. Asthma is a chronic airway disease associated with clusters of respiratory symptoms, including wheezing and dyspnea. Asthma is a common disease with different prevalences according to ethnicity that range from approximately 7% to 18.0% 1. The incidence of asthma in the adult population (>20 years old) was estimated to be approximately 3.63-6.07 per 1,000 people in Korea 2. Because the diagnosis of asthma is based on the functional deterioration of the lower airway, the underlying pathophysiologic causes of asthma have been reported to vary among asthmatic patients 3. According to the pathophysiologic mechanisms, asthma is classified as two major endotypes of T-helper type 2 cell (TH2)-high and TH2-low diseases 4. Thus, in addition to TH2-high related diseases, such as allergic rhinitis, other immune-related diseases could complicate asthma. In addition, both genetic factors and environmental triggering factors have been suggested to be related to asthma 5,6. Migraine disorder is another chronic disease characterized by multiple symptoms of headache and/or aura. As many as approximately 1.04 billion people suffer from migraines worldwide (95% uncertainty interval = 1.00-1.09) 7. In Korea, approximately 6.0% of the adult population (19-69 years) suffers from migraines 8. Migraines are characterized by recurrent episodic symptom sequences of a premonitory phase, an aura phase, a headache phase, and a postdrome phase 9. The environmental triggering factors can hyper-activate the hypothalamic-pituitary-adrenocortical axis and autonomic nervous system in migraine susceptible subjects 10. These hormonal and nervous changes induce vasodilation, the release of vascular factors, including growth factors, cytokines, nitric oxide, norepinephrine and calcitonin gene-related peptide, and interaction with endothelial cells, res...