We are now detecting an increasing number of solitary pulmonary nodules (SPNs) that are difficult to diagnosis. The purpose of this study was to evaluate the useful radiologic findings for differentiating benign from malignant pulmonary nodules. Material and Method: The high-resolution CT (HRCT) findings of SPNs smaller than 3 cm in largest diameter were evaluated in 134 patients with malignant and benign nodules in regard to internal structures, margin characteristics, and surrounding parenchymal responses. Result: The nodules with the area of ground-glass attenuation (GGA) greater then 50% were noted in adenocarcinoma, inflammatory lesions and some of metastatic tumors, and the lesions greater than 90% were noted only in adenocarcinoma. The area of GGA in non-adenocarcinoma, benign tumors and tuberculomas were less than 50%, and mainly less than 10%. The findings of air bronchogram, spiculation, lobulation, vascular involvement, and pleural indentation were some noted at every types of malignant tumors, but especially high over than 30% in adenocarcinomas. Conclusion: Most peripheral lung adenocarcinomas form a characteristic radiologic findings especially in HRCT. Evaluation of these findings would be helpful in differentiating between lung cancer, especially adenocarcinoma, and other lesions.