Irradiation of the axillary nodes is often indicated in the clinical radiotherapy of patients with lymphoma and breast cancer. The relative location of the axillary nodes and the humeral head has historically been estimated based on lymphangiographic data. Since CT offers more precise definition, the axillary nodal region in relation to the humeral head was systematically studied in 61 patients undergoing CT simulation for breast conservation therapy. Differences in arm position (degree of abduction) significantly affected the location of the axillary nodal region in relation to the humeral head. With the arm abducted beyond 55 degrees, humeral head blocking would result in blocking of some of the axillary nodes.