Twenty patients with Stage I or II breast cancer, all of whom had undergone radiation therapy planning, were examined with internal mammary lymphoscintigraphy (IMLS) and computed tomography (CT). Based on the results of IMLS, radiation fields were revised in 12 cases (60%). The mean number of nodes identified by IMLS in each patient was 7.8, which is in agreement with previously published autopsy and scintigraphic data. CT identified 243 possible nodes of normal size, but only 49 of them were within 10 mm of regions shown to be positive on the scintigram. The authors conclude that IMLS is the method of choice for defining parasternal lymphatic drainage and identifying those internal mammary nodes that are normal in both size and function.