This study covers 40 cases with the diagnosis of nonsmall cell lung cancer in our thoracic surgery clinic between November 1994 and April 1995 on whom mediastial lymph node dissection was performed. From 40 cases 724 lymph nodes were removed ranging between 1 mm and 35 mm in diameter by taking into account the smallest diameter of a node. The lymph nodes removed were numbered according to the map of ATS (American Thoracic Society). Their diameters were measured and examined histapathologically. They were classified in groups with the upper limits of 10, 15 and 20 cm according to their diameters. Within 106 nodes removed (14.6%), metastasis was detected. Nodes with metastases 45.5 % were above 10 mm, 15 % were above 15 mm and 5.7% were above 20 mm in diameter. By examining 724 lymph nodes, and classifying them as groups of 10 mm, 15 mm and 20 mm according to their diameters, sensitivity of preoperative imaging technics in determining metastasis to mediastinal lymph nodes were found 88%, 86%, and 86% respectively. Specificity were found 22%, 50% and 46% respectively. As a result, in patients with non-small cell lung carcinoma it is not appropriate to decide if the patient is in N2 stage by measuring the diameter of node using imaging technics in preoperative evaluation of mediastinal lympadenopathy. So far, for the correct staging treatment and prognosis of the disease in patients qith mediastinal lymphadenopathy, mediastinal exploration should be done without taking care of the diameter of mediastinal node.