Yoshiaki Kitaguchi,1,2 Keisaku Fujimoto,3 Masayuki Hanaoka,1 Takayuki Honda,4 Junichi Hotta,2 Jiro Hirayama2 1First Department of Internal Medicine, Shinshu University School of Medicine, Matsumoto, 2Department of Internal Medicine, Okaya City Hospital, Okaya, 3Department of Clinical Laboratory Sciences, 4Department of Laboratory Medicine, Shinshu University School of Medicine, Matsumoto, Japan Background: The syndrome of combined pulmonary fibrosis and emphysema (CPFE) is a recently described entity associating upper-lobe emphysema and lower-lobe fibrosis. We sought to evaluate differences in pulmonary function between CPFE patients with and without airflow obstruction. Subjects and methods: Thirty-one CPFE patients were divided into two groups according to the presence or absence of irreversible airflow obstruction based on spirometry (forced expiratory volume in 1 second/forced vital capacity <70% following inhalation of a β2-agonist) as follows: CPFE patients with airflow obstruction (CPFE OB+ group, n=11), and CPFE patients without airflow obstruction (CPFE OB– group, n=20). Pulmonary function, including respiratory impedance evaluated using impulse oscillometry and dynamic hyperinflation following metronome-paced incremental hyperventilation, was retrospectively analyzed in comparison with that observed in 49 chronic obstructive pulmonary disease (COPD) patients (n=49). Results: In imaging findings, low-attenuation-area scores on chest high-resolution computed tomography, representing the degree of emphysema, were significantly lower in the CPFE OB– group than in the CPFE OB+ and COPD groups. In contrast, the severity of pulmonary fibrosis was greater in the CPFE OB– group than in the CPFE OB+ group. In pulmonary function, lung hyperinflation was not apparent in the CPFE OB– group. Impairment of diffusion capacity was severe in both the CPFE OB– and CPFE OB+ groups. Impulse oscillometry showed that respiratory resistance was not apparent in the CPFE OB– group compared with the COPD group, and that easy collapsibility of small airways during expiration of tidal breath was not apparent in the CPFE OB+ group compared with the COPD group. Dynamic hyperinflation following metronome-paced incremental hyperventilation was significantly greater in the COPD group than in the CPFE OB– group, and also tended to be greater in the CPFE OB+ group than in the CPFE OB– group. Conclusion: The mechanisms underlying impairment of physiological function may differ among CPFE OB+ patients, CPFE OB– patients, and COPD patients. CPFE is a heterogeneous disease, and may have distinct phenotypes physiologically and radiologically. Keywords: CPFE, COPD, respiratory impedance, dynamic hyperinflation