Asthma and bipolar disorder (BD) are 2 distinct diseases that share similar pathophysiology. This study aimed to determine their relationship thorough a meta-analysis of articles on their comorbidity rate. The aim of the study is to examine the overall prevalence rate of BD in asthmatic patients and of asthma in BD patients compared to healthy controls. Electronic research of PubMed and ClinicalTrials.gov was performed. Articles discussing the prevalence rate of BD in patients with/without asthma and the prevalence rate of asthma in those with/without BD, as well as clinical trials in humans and case-controlled trials or cohort studies, were all included. Case reports or series and nonclinical trials were excluded. Through a random-effects model, a meta-analysis of the results of 4 studies comparing the prevalence rate of BD in patients with/without asthma, and in 6 studies comparing the prevalence rate of asthma in subjects with/without BD were performed. There were significantly higher prevalence rates of BD in asthmatic patients than in healthy controls (P < 0.001) and of asthma in BD patients than in healthy controls (P < 0.001). Only the patient's mean age significantly modulated the odds ratio of the prevalence rate of asthma in BD patients (slope = 0.015, P < 0.001). Only 10 studies were included and most were cross-sectional studies. The possible confounding effect of medication on BD or asthma onset was not investigated. Any possible etiology of the comorbidity was also not determined. This meta-analysis highlights the importance of the significantly high comorbid rate of BD and asthma, and the positive association with age. Special attention must be given to the comorbidity of asthma and BD, especially in older patients.