Purpose: We retrospectively investigated the pathological diagnoses of pulmonary lesions presenting as pure ground-glass opacities (GGOs) to evaluate the risk of invasive malignancy.Methods: We examined 191 GGO lesions, including 114 pure GGO and 77 mixed lesions, in 160 patients who underwent resection between January 2008 and December 2010.Results: Of the 114 pure GGO lesions, 14 (12%) were diagnosed as invasive lung cancer and 16 (14%) as minimally invasive adenocarcinoma. Twenty-one lesions exhibited pleural indentation on high-resolution computed tomography (HRCT), and 5 of these were diagnosed as invasive cancer, indicating an invasive tendency of pure GGO lesions with pleural indentation (odds ratio, 2.64). Of 14 pure GGO lesions positive on positron emission tomography (PET), 8 were diagnosed as invasive lung cancer, indicating an invasive tendency of pure GGO lesions with PET positivity (odds ratio, 16.0; p <0.001; sensitivity, 67%; specificity, 89%).Conclusion: Invasive lung cancer accounted for 12% of the pure GGO lesions. Pure GGO lesions should be carefully monitored by periodic chest computed tomography, and surgical resection is recommended when they exhibit pleural indentation on HRCT or positivity on PET.