The aim of this study is to classify patients into risk groups for mediastinal lymph node metastases. Three hundred and thirty-seven patients underwent lung resection for lung cancer. The nodal status was pN0 in 181 patients, pN1 in 62 and pN2 in 94. The presence of the involvement of one mediastinal compartment (superior or inferior) or two mediastinal compartments (superior and inferior) was considered to be the main end point. One mediastinal compartment was involved in 65 patients and two mediastinal compartments in 29 patients. Two variables (visceral pleural invasion and the primary tumor location) were retained in the model. The regression tree analysis categorized patients into 3 risk groups for the involvement of two mediastinal compartments. The low-risk group included 118 patients with a tumor located in the left side and no visceral pleural invasion. The intermediate-risk group included 160 patients with a tumor located in the right side and no visceral pleural invasion. The high-risk group included 59 patients with visceral pleural invasion and a tumor located in the right side or left lower lobe. A practical, easy-to-use risk grouping system is proposed to aid the decision making and to simplify mediastinal lymphadenectomy procedure.