To investigate whether the assessment of resectability of lung cancer can be reliably made on the basis of computed tomography (CT), the thoracic CT scans of 96 lung cancer patients who were operated on, and in whom the tumour was classified to be of stage III at preoperative CT or at thoracotomy, were analysed. Of the patients, 58 underwent complete resection of the tumour, whereas thoracotomy resulted in non-complete resection or exploration only in 38 cases. Overlapping of the CT findings in the groups of complete and non-successful resection was observed. The majority of tumours with apparent growth to the carina, trachea, oesophagus or great vessels at CT were completely resected. Tumours that only bordered on the pleura could not be distinguished from those with pleural invasion. Mediastinal lymph node enlargement did not always mean metastatic spread. We conclude that there is no definite sign that identifies non-resectability of lung tumours, and a certain proportion of non-curative thoracotomies must be accepted.