There is some evidence that breathing retraining may be beneficial for patients with asthma, but the mechanism behind this benefit is still unknown. One hypothesis is that individuals can be trained to raise carbon dioxide levels and thereby reverse the bronchoconstrictive effects of hypocapnia and utilize the bronchodilatory effects of hypercapnia. This theory presupposes that individuals with asthma have lower carbon dioxide levels than the healthy population. This article reviews the available evidence supporting the hypothesis and concludes that although attractive, there is currently insufficient evidence to attribute the benefits of breathing retraining to this mechanism.