To determine the regional accuracy of computed tomography of the mediastinum in staging lung cancer, we compared the results of preoperative computed tomographic staging to pathologic findings in lymph nodes taken at mediastinoscopy and/or thoracotomy in 61 patients. Twenty-two patients had adenocarcinoma, 24 had squamous cell carcinoma, eight had large cell tumors, and seven had small cell cancer or mixed cellular types. Sixteen patients had Stage I, eight had Stage II, and 37 had Stage III disease. Thirteen patients had mediastinoscopy only, and the remaining 48 patients had thoracotomy. Computed tomographic staging of the mediastinum as a whole had an accuracy of 88% with a negative predictive index of 96.1%. In examining the differential regional accuracy within the mediastinum we found results in the aortopulmonary window to be inferior to those of other regions, with an accuracy of 80% and a negative predictive index of 83.3%. The reliability of computed tomographic scan staging varied relative to cell type. The accuracy rate in adenocarcinoma was 94.7% compared to 70.6% in squamous cell carcinoma. Computed tomography is accurate for staging the mediastinum in lung cancer, and this accuracy holds over the regions of the mediastinum except the aortopulmonary window. Computed tomography is more accurate for staging adenocarcinoma than squamous cell cancer.