Abstract:  Purpose: It is supposed that a diameter of a pulmonary mass on deep expiration computed tomography (CT) is greater than that of on deep inspiration CT. The purpose of the study is to evaluate usefulness of changes in mass size between on breath holding expiration and on breath holding inspiration CT for differential diagnosis of pulmonary masses. Patients and Methods: CT was performed prospectively both with deep inspiration and with deep expiration in 48 patients, 78 masses with 19 primary pulmonary cancers, 19 metastatic lung cancers, 25 active inflammatory pulmonary lesions and 15 old inactive inflammatory lesions, using 16-detector-row CT unit. Length, width and height of a lesion were measured on both axial and reconstructed coronal images. Expansion rate was defined as [(product of length, width and height of a mass at deep expiration) – (those of at deep inspiration)]/ (those of at deep inspiration). Results: Mean ± one standard deviation of expansion rate was 13.2 ± 9.9 in primary pulmonary cancers, 13.7 ± 8.8 in metastases, 12.9 ± 13.4 in active inflammatory lesions and -3.5 ± 10.9 in old inactive inflammatory lesions. Expansion rate of old inflammatory inactive lesions was significantly less than that of others (p < 0.001). The rate of upper pulmonary lobar lesions, that of middle lobar lesions and lower lobar lesions were 13.1 ± 10.5, 16.9 ± 13.3 and 11.0 ± 10.5, respectively. They are significantly the same. Conclusion: Expansion rate was useful for differentiation of active inflammatory lesions from inactive lesions.