Purpose : To evaluate factors influencing the CT assessment of mediastinal lymph node metastasis in patientswith non-small cell lung cancer. Materials and Methods : CT scans of 198 patients who had undergone thoracotomyand mediastinal lymph node dissection for non-small cell lung cancer were retrospectively evaluated using a sizecriterion of $geq$ 10mm in the short axis. To evaluate the accuracy of CT in diagnosing lymph node metastasis on anodal station-by-station basis, CT and pathological results were correlated. Analysis included a comparison of thesensitivity and specificity of CT according to 1) cell type of tumor, squamous cell carcinoma versusadenocarcinoma (excluding bronchioloalveolar cell carcinoma) ; 2) histologic differentiation;3) tumor size;4)central and peripheral of the tumor;5) the presence or absence of obstructive pneumonitis and/or atelectasis;6)the presence or absence of prior granulomatous disease. Results : The overall sensitivity, Specificity, positivepredictive value, and negative predictive value of CT in diagnosing mediastinal lymph node metastasis were 65%,84%, 43%, and 93%, respectively. Sensitivity for squamous cell carcinoma (72%) was significantly higher than thatfor adenocarcinoma(44%)(P<0.01). Higher specificities were noted in patients without obstructive pneumonitisand/or atelectasis(91% versus 75%)(P<0.01), and with a peripherally located tumor (90% versus 82%)(P<0.01).sensitivity and specificity were not appreciably altered by other variables. Conclusion : In the CT assessment ofmediastinal lymph node metastasis the cell type of adenocarcinoma adversely affected sensitivity, with a highfrequency of normal-sized metastatic nodes. Obstructive pneumonitis caused by central tumor adversely affectedspecificity with the frequent occurrence of hyperplastc nodes.