BACKGROUND Clinical stage (c-stage) I non-small cell lung cancer (NSCLC) is generally indicated for surgery, however, surgical exploration sometimes reveals advanced disease, thus resulting in incomplete resection. METHODS A total of 645 consecutive patients were investigated in which 347 were diagnosed to have c-stage IA in 347 and 298 were diagnosed to have IB disease. All cases underwent operation and were investigated for resectability and the cause of an incomplete resection. RESULTS The c-Stage IA patients included 16.6% of T3/4 and 10.4% of N2 whereas clinical stage IB patients included 14.4% of T3/4 and 18.8% of N2/3. A complete resection was performed in 594 patients (91%). In 347 c-stage IA patients, the complete resection rates were 93% in adenocarcinomas (235/252), 100% in squamous cell carcinomas (76/76), and 89% in others (17/19). In 298 c-stage IB patients, the complete resection rates were 86% in adenocarcinomas (141/164), 90% in squamous cell carcinomas (90/100), and 94% in others (31/33). The 5-year survival rates of the c-stage IA and IB patients who underwent a complete resection were 66.4 and 48.3%, respectively. However, the same rates were 18.4 and 14.7% for c-stage IA and IB patients who underwent an incomplete resection. The reasons for an incomplete resection in 54 patients were malignant pleurisy in 38 (70.4%), extranodal invasion of mediastinal nodal metastasis in ten (19%), an incomplete bronchial margin in three (5.6%), and ipsilateral pulmonary metastases in two (3.7%), and ipsilateral adrenal metastasis in one (1.3%). In 13% of the c-stage IB adenocarcinomas, pleural metastasis was discovered during thoracotomy. CONCLUSIONS Pleural dissemination was the most frequent cause of an incomplete resection, and its prevalence was high in c-stage IB adenocarcinomas.