From 1974 to 1984, 225 patients underwent thoracotomy at Memorial Sloan-Kettering Cancer Center for primary non-small cell lung cancer invading only the mediastinum (T3). The perioperative mortality was 2.7 per cent, and the nonfatal complication rate 13 per cent. Forty-nine patients underwent complete resection of all intrathoracic disease, with a median survival of 17 months, 3-year survival of 21 per cent, and 5-year survival of 9 per cent. Thirty-three patients underwent pulmonary resection with simultaneous iodine-125 interstitial implantation or iridium-192 delayed afterloading to areas of unresectable primary or nodal disease, with a median survival of 12 months, 3-year survival of 22 per cent, and 5-year survival of 22 per cent. One hundred and one patients underwent interstitial implantation without resection, with a median survival of 11 months, 3-year survival of 9 per cent, and no 5-year survivors. Forty-two patients had incomplete resection without intraoperative radiation therapy and fared no better than a cohort group of 44 unoperated patients with clinical evidence of mediastinal invasion--both groups had a median survival of 8 months and no 3-year survivors. An aggressive surgical approach with pulmonary resection and/or brachytherapy appears to offer some survival advantage to this group of patients. In particular, 5-year survival rates ranging from 7 to 15 per cent were observed in subsets of intraoperatively treated patients with invasion of pulmonary vein, phrenic nerve, esophagus, or pericardium and in those with clinically occult T3 disease.