Purpose: To evaluate the role of mediastinoscopy and CT in the preoperative nodal evaluation in patients with T1 lung cancer. Materials and Methods: Between November 1994 and July 1996, 125 patients underwent thoracotomy and/or mediastinoscopy for surgical treatment of lung cancer. Among them, 35 patients had T1 lung cancer (peripheral lung cancer less than 3cm in diameter) on CT. One patient finally proved to have T4 lung cancer with pleural seeding at thoracotomy. In the remaining 34 patients, pathologic evaluation of mediastinal lymph node metastasis was feasible and the results were correlated with CT findings. On CT, nodes larger than 10mm in short-axis diameter were regarded as abnormal. Results: The patients had adenocarcinoma in 12, squamous cell carcinoma in 11, bronchioloalveolar carcinoma (BAC) in 10, and large cell carcinoma in one. Fifteen among total 478 sampled lymph nodes contained malignant tumor. Six (three with adenocarcinoma, two with squamous cell carcinoma, and one with large cell carcinoma) of 34 patients (18%) had nodal metastasis. With 112 sampled nodes, BAC did not show any nodal metastasis. Sensitivity and specificity of CT for nodal detection were 0% and 100% for 2R, 0% and 100% for 4R, 100% and 97% for 5, 50% and 100% for 7 and 0% and 100% for 10R, respectively. Conclusion: T1 lung cancer shows relatively high (18%) prevalence of mediastinal lymph node metastasis. Because small nodes less than 10mm in diameter contain malignancy and CT is insensitive in detection of metastatic nodes, mediastinoscopy is still needed for preoperative nodal evaluation except BAC.