Abstract Parasternal lymphoscintigraphy is an easily performed technique for visualizing and accurately determining thethree dimensional location of internal mammary lymph nodes. Sixty-eight women with primary or locally recurrent breast cancer underwent parasternal lymphoscintigraphy before treatment planning. The mean depth and lateral position of the internal mammary nodes were 1.8. ± 0.9 cm and 2.4 ± 0.9 cm respectively. Thirteen per cent of the nodes would have been undertreated by the standard tangential technique used at our institution. Six of 15 patients whose treatment plans were examined retrospectively had at least 1 lymph node undertreated by the standard tangential tec hnique. Thus, although standard tangential portals provide adequate coverage for the internal mammary lymph nodes in a majority of patients, parasternal lymphoscintigraphy can insure complete coverage of this lymph node group.