In mediastinal dissection through a right thoracotomy, the definition of station 2 is arbitrary because no anatomical landmark indicates the line drawn tangentially to the upper margin of the aortic arch. We have developed a technique to localize it by evaluating the distance between the upper aortic arch and the azygos vein on a CT scan. This distance located intraoperatively above the azygos vein permits the surgeon to draw an imaginary line parallel to the azygos vein, which we consider to be the limit between station 2 and station 4. To verify the precision of the technique, an 8-mm clip was positioned at the intersection between the imaginary line dividing station 2 and station 4 and the superior vena cava in 38 consecutive right-sided lateral muscle-sparing thoracotomies. The definition of the station 2/4 limit was defined as "excellent" if the upper aortic arch line crossed the clips, "good" if clips were </= 5mm from it, "acceptable" if the distance was 6-10 mm, and "poor" if the distance was higher than 10 mm. Clip evaluation was possible in 37 patients. The definition of the limit between station 2 and station 4 was excellent in 26 cases (70.2%), good in 10 cases (27%), acceptable in no case and poor in 1 case (2.7%). In this latter case,the presence of a lusory artery was probably responsible for the poor clip positioning. This simple technique improves the precision of lymph node staging during mediastinal dissection,providing an excellent or good definition in more than 95% of patients submitted to lateral thoracotomy. Further studies are needed to verify whether the technique is applicable with the same precision during postero and antero-lateral thoracotomy.