Background and purpose: An accurate internal mammary (IM) lymph node localization technique is required for proper irradiation of the IM lymph nodes in breast cancer patients. In this study the measurement accuracy of three techniques for direct or indirect localization of the IM nodes was estimated. Materials and methods: In 40 patients the IM lymph node depth and lateral distance from the patient midline were measured with lymphoscintigraphy in intercostal spaces 2, 3 and 4. The corresponding position of an IM vessel was measured with sonography and CT in intercostal spaces 1-4. The sonography and CT vessel measurements in the four intercostal spaces were compared to determine the measurement accuracy of sonography. The node and vessel data in intercostal space 2 were inserted into a mathematical model to determine the measurement accuracy of lymphoscintigraphy and CT for node detection. Results: Vessel depths measured by sonography were systematically too shallow and the lateral vessel position could not be accurately determined. The mathematical model showed that the node depth and lateralization in one intercostal space can be measured directly by lymphoscintigraphy within an accuracy (1 SD) of 5 mm in depth and 6 mm in the lateral direction. The accuracy of CT for indirect node detection was 6 mm in depth and 7 mm in the lateral direction. Conclusions: Sonography is not a suitable technique for measuring the IM vessel or node position. Lymphoscintigraphy and CT have measurement accuracies for node detection that are acceptable for radiotherapy. © 2002 Elsevier Science Ireland Ltd. All rights reserved.