We examined the characteristics of lymph nodes metastases to the mediastinum on cases of two-hundred-consecutive operations for primary lung cancer from October of 1987 to May of 1990, comparing the intraoperative macroscopic diagnosis of lymph nodes to the histologic reports. The purpose of this study is to know how important the mediastinal dissection is and how many cases resulting in vain are hidden behind it. Thirty three cases were excluded because of limited operation or other reasons. Remaining a hundred sixty seven cases were divided on the basis of lymph node metastasis into following 5 groups. Forty three cases (26%), of which N2 disease was correctly diagnosed macroscopically, true positive for N2, were classified into group A. Sixteen cases (10%), group B, in which we failed to detect N2 positive from dissected specimens during operation, was considered to be false negative for N2, thirty one cases of N1 disease (18%), were classified into group C and thirteen cases of N0 disease (8%) into group D with false positive of lymph node prediction for metastasis. Sixty four cases of N0 disease (40%) were classified into group E, true negative for N0 prediction, which were correctly detected by macroscopic appearance of lymph nodes. Nine cases had only mediastinal lymph nodes metastases without hilar and lobar lymph node. Almost of them with this skip phenomenon occurred in the patients with adenocarcinoma in the upper lobe. It could be concluded that we dissected the mediastinal lymph nodes in this series, salvaging one N2 disease of ten cases and wasting our efforts four cases of those cases, in vain.