The purpose of this study was to describe the CT findings of focal organizing pneumonia and to compare the findings with pathology. CT findings of histologically proven focal organizing pneumonias in 26 consecutive patients were analyzed. In 17 patients who had undergone surgical resections, the findings were correlated with pathology. Focal organizing pneumonias appeared as a nodule (n= 13) or a mass (n=13), ranging from 9 mm to 66 mm in diameter. Ground-glass opacity was seen in 6/13 (46%) nodules and 6.5/13 (50%) masses (k=.48) with an extent ranging from 5% to 75% (mean, 16%). In 4/26 (15%) patients, the extent was more than 50% of the lesion. They showed smooth (n=4), lobulated (n=8), spiculated (n=1), or lobulated and spiculated margin (n=13). On correlative analysis, nodule or mass on CT consisted histologically of intraalveolar exudate or microabscess, chronic inflammatory cell infiltration, fibrotic nodules, and polypoid granulation tissue in the alveolar or bronchiolar spaces. Ground-glass opacity consisted of interstitial fibrosis and chronic inflammatory cell infiltration and intraalveolar polypoid granulation tissue. Focal organizing pneumonia may simulate a lung cancer with variable appearances on CT and the findings reflect underlying histopathology of the disease.