Using a size criterion of 1 cm or greater as evidence for abnormality, the size of mediastinal lymph nodes identified by computed tomography (CT) was a poor predictor of mediastinal lymph node metastases in a series of 86 patients who had surgery for bronchogenic carcinoma. The surgery included full nodal sampling in all patients. Of the 86 patients, 36 had nodes greater than or equal to 1 cm identified by CT. Of the 21 patients with mediastinal metastases proven at surgery, 14 had nodes greater than or equal to 1 cm (sensitivity = 67%). Of the 65 patients without mediastinal metastases, 22 had nodes greater than or equal to 1 cm. Specificity was 66% (43/65). Obstructive pneumonia and/or pulmonary collapse distal to the cancer was present in 39 patients (45%). Of these, 21 had mediastinal nodes greater than or equal to 1 cm; 10 harbored metastases and 11 did not. Obstructive pneumonia and/or pulmonary collapse is a common occurrence in bronchogenic carcinoma, but mediastinal nodes greater than or equal to 1 cm in this circumstance cannot be presumed to represent metastatic disease. Metastatic mediastinal lymph node involvement was related to nodal size also in patients with evidence of prior granulomatous disease and in patients with no putative benign cause for nodes greater than or equal to 1 cm. In both of these groups, metastatic nodal disease was found in only 25% of nodes greater than or equal to 1 cm.