Although there have been several attempts in dividing N2 patients into several subgroups on the basis of different prognoses, the correct treatment for these patients is still a moot point. Even multimodal treatment, which is the most common therapy used, does not result in a consistent outcome. The aim of our study is to assess the prognostic value of the extent of mediastinal lymph node infiltration in surgically treated non-small cell lung cancer (NSCLC). From January 1990 to December 1997, 682 patients underwent surgery for NSCLC at the Thoracic Surgery Unit, University Hospital of Siena, 87 of which (12%) had mediastinal involvement. Studies on the number of lymph node stations show that those with one station involved tend to have a better 5-year survival rate with respect to the others. We studied the number of lymph node stations by using a new critique based on the percentage of lymph node infiltration. The percentage is obtained from a ratio of the number of involved nodes to the total number of nodes removed. The result was an improved 5-year survival ratio in patients with lymph node infiltration, lower than 50% with respect to the others, and the difference was significant (P=0.0001). It appears that surgery may be the most suitable option for treating those N2 patients that we consider to be in 'early N2 phase', in view of long term survival. Although an invasive technique like mediastinoscopy seems to be the appropriate indicator in selecting N2 patients, it does not allow the calculation of the ratio a priori.