In order to evaluate the reliability of thoraco-mediastinal CT in the preoperative evaluation of primary lung cancer, regarding "N" (lymph nodes) parameter, we compared CT data with those obtained at histopathology of mediastinal lymph nodes. We re-examined 130 patients who had undergone lobectomy or pneumonectomy combined with mediastinal node dissection. CT criterion of neoplastic nodal involvement is morphological, based on size of the node as related to its location. CT is very sensitive in evaluating both normal and pathological nodes but not likewise specific; in fact, it does not allow differential diagnosis between neoplastic and phlogistic causes. This limitation must be kept in mind in the preoperative evaluation of the "N" parameter. Moreover, CT findings of mediastinal involvement on the opposite side (N3) must be confirmed with mediastinoscopy or CT-guided biopsy before ruling radical surgery out.