Hyperventilation-induced hypocapnia is common among asthma patients. This case study illustrates both methodology and results from a patient undergoing training in capnometry-assisted respiratory training (CART). CART is a 4-week training aimed at normalizing basal and acute levels of end-tidal carbon dioxide (PCO(2)) using a portable capnometer. In the presented case, basal levels of PCO(2) increased from hypocapnic to normocapnic range over the course of treatment. Improvements were accompanied by improvements in lung function and reductions in diurnal lung function variability. Improvements remained stable throughout follow-up.