Computed tomography and the TNM classification of lung cancer. Computed tomography (CT) of the thorax and upper abdomen was prospectively evaluated in 84 patients with potentially operable lung cancer. Invasion into the thoracic wall and the mediastinal structures was not accurately demonstrated by CT. For metastatic mediastinal lymph nodes, the sensitivity and specificity of CT were, respectively, 86% and 61% and the positive and negative predictive indices 49% and 91%. For T1, T2 and T3 tumours the negative indices were 100%, 96% and 71%. Positive predictive index did not differ between squamous cell carcinoma and adenocarcinoma. Adrenal metastases were CT-suspected in 17 cases and liver metastases in eight, but were verified by ultrasonography in only one and four cases. CT should be used in preoperative investigation of lung cancer, irrespective of stage. Demonstration of thoracic-wall or mediastinal invasion need not exclude tumour resection. Preoperative mediastinoscopy is indicated if CT shows nodal metastases or if there are signs of tumour invasion, but not in CT-negative T1 or T2 tumour. Abdominal metastases indicated by CT should be investigated with CT-guided needle biopsy. 