Conservative surgery combined with radiation therapy for the treatment of early breast carcinoma has been shown to achieve both a high rate of local tumor control and good cosmetic results with a minimum of complications. Whether the internal mammary lymph nodes (IMNs) should be included in the treatment volume is a topic of considerable controversy. Radionuclide internal mammary node lymphoscintigraphy (IMN-LS) can locate these nodes in three dimensions. We have analyzed the results of IMN-LS in 167 patients imaged at the Dana-Farber Cancer Institute and treated at the Joint Center for Radiation Therapy between 1977 and 1980. The location of the IMNs was found variable from patient to patient. At least one IMN was not included within tangential fields arbitrarily arranged to have a medial entrance point 3.0 cm across the midline in 17% of evaluable patients. However, 48% and 66% of patients had IMNs that could be adequately treated with fields positioned only 1.0 cm or 2.0 cm across midline, respectively. We conclude that when treatment of the IMNs is warranted, IMN-LS not only assures their complete coverage in the majority of patients but also may help reduce the amount of heart and lung irradiated.