To clarify the differences in radiological findings between primary lung cancers and benign nodules measuring less than 10mm, we examined chest computed tomography (CT) findings. Of 82 patients with solitary pulmonary nodules less than 10mm in diameter who had undergone surgical biopsy, 21 patients with primary lung cancer and 45 patients with benign lesions (9 patients with tuberculosis, 12 with non-specific inflammation, 10 with benign lung tumor, 10 with intrapulmonary lymph nodes and 4 with others) were examined. Seven patients with atypical adenomatous hyperplasia and nine patients with metastatic lung cancer were excluded. Primary lung cancers had an ill-defined tumor margin and spiculation significantly more frequently than benign nodules (P<0.01). Involvement of bronchi or vessels was observed significantly more frequently in primary lung cancers than in benign nodules (P<0.05), while pleural indentation did not show significant differences in frequency. Retrospective chest X-ray or CT films were reviewed for seven patients with primary lung cancers and 12 with benign nodules, with a mean interval of 24+/-17 months. Primary lung cancers enlarged or appeared as new nodules more frequently than benign lung nodules (P<0.05). Among 17 lung cancer patients who underwent mediastinal lymph node dissection, the cancer was at a more advanced stage than T1N0M0 in four (24%). We conclude that ill-defined margins, spiculation, involvement of bronchi or vessels, and tumor enlargement visualized by CT are still important signs of malignancy even for nodules less than 10mm in size. Tumor size, even for lung cancers measuring less than 10mm, is not an indication for limited resection.