We investigated the relationship of lymph node metastasis to primary tumor size and microscopic appearance in 92 resected specimens obtained from patients with roentgenographically occult lung cancer (ROLC) located at a site along the airway between the main bronchus and the sub-subsegmental bronchi. Most of the patients were discovered by mass screening. All were treated surgically after bronchoscopic localization of cancer. The bronchial tree of the resected specimens was serial-sectioned into 2-mm thick blocks from the margin of resection to the sub-subsegmental bronchi. Bronchial wall invasion was noted in some blocks of all the specimens. The length of longitudinal extension (LLE) was defined as the product of the thickness and the number of consecutive blocks involved, counting from the most proximal to the most distal block. LLE was used as primary tumor size. Hilar and mediastinal lymph nodes were examined in 84 patients who underwent lymph node dissection. No nodal involvement was found in 59 cancers with LLE of less than 20 mm. Of 25 cancers with LLE of 20 mm or more, six showed nodal involvement. Eleven in situ carcinomas and four cancers of the "suspicious for invasion" type showed no lymph node metastasis. We contend that no lymph node dissection is required when pulmonary resection is performed for patients with ROLC if it is in situ carcinoma, if it is of the "suspicious for invasion" type, or if the LLE is smaller than 20 mm.