Ground-glass opacity (GGO) attracts attention because of the possibility of early lung cancer. However, some lesions are reduced in size or disappear at follow-up. This study was designed to explore the natural history of solitary GGO, to determine the prevalence of malignancy and to identify factors predictive of benignity or malignancy. Solitary and focal GGO lesions [pGGO (p=pure) and mGGO (m=mixed) based on the presence of a solid component] of less than 3 cm were included. Lesions of less than 1cm were followed up by chest HRCT 3 months later and lesions over 1cm were investigated by percutaneous needle biopsy (PCNB). One hundred and eighty-six patients (69 pGGO and 117 mGGO) were enrolled. Of the 69 pGGO lesions, 7 were diagnosed as pre-malignant or malignant lesions, 3 as benign lesions and 26 pGGO lesions (37.6%) were reduced or disappeared (transient lesions) at follow-up chest HRCT. The other 33 lesions showed no significant change during follow-up. Thus, the probability of malignancy in pGGO was 7/36 (19.4%). On the other hand, of the 117 mGGO lesions, 26 were found to be malignant, 3 were diagnosed as benign and 57 lesions (48.7%) were reduced or had disappeared at follow-up chest HRCT. The other 31 lesions showed no change during follow-up, and thus the probability of malignancy in mGGO was 26/86 (30.2%). A female sex and a spiculated mGGO border were found to be related with malignancy. However, a high blood eosinophil count was strongly associated with regressing or transient mGGO, suggesting that pulmonary infiltrate with eosinophilia (PIE) might have been responsible. We recommend short-term follow-up by chest HRCT be conducted for mGGO lesions in the presence of high eosinophilia--regardless of lesion size.