Objective To review the serial CT findings of cryptogenic organizing pneumonia(COP)and semiquantitatively analyze the changes after corticosteroid therapy.Methods The clinical and radiological features of 13 patients with COP confirmed by pathology were retrospectively collected and analyzed.The lung lesions extent on CT scans was estimated using semi-quantitative method.Changes of overall disease extent were evaluated by comparing the initial CT images and the follow-up CT scans after corticosteroid therapy.Results CT images of all cases revealed diffuse and bilateral lesions,mainly located in lower zone,distributed as predominantly subpleural and(or)along the bronchovascular bundle.The two most common manifestations of lung abnormality on initial scans were ground-glass opacity(GGO)(13 cases)and consolidation(12 cases),the average areas of which were 13％ and 10％ respectively.The treatment with antibiotics was ineffective.After treatment with corticosteroid,the lesions resolved completely in 2 patients ; the disease was decreased in extent in 8 patients ; and the lesion showed no change in extent in one patient,while the disease progressed in extent in 2 patients on follow-up CT.The most common CT findings were GGO(10 cases)and reticulation(5 cases),followed by consolidation(4 cases),the average areas of which were 19％,2％ and 1％ respectively.Conclusion The CT features of the patients with COP are characteristic.The most lesions resolved or improved after corticosteroid therapy on follow-up CT scans.The estimation of disease extent with semi-quantitative method is helpful for the clinicians to evaluate the therapeutic effect.