The authors discuss the value of ultrasound in determining the depth of the internal mammary vessels und their distance from the medial line. Real-time imaging with a high frequency transducer and a pulsed Doppler system gave highly accurate results. Depending on the intercostal space, the depth of internal mammary vessels varies from 17 to 22 mm. The distance of the internal mammary vessels from the medial line is a function of the intercostal space and the body side, and varies from 25 to 33 mm. Results were found to vary as a function of patient height and weight; likewise, mastectomy was seen responsible for a modification of 2 mm in measurements. Precise localization of the internal mammary vessels allows optimization of radiotherapy, and this rapid examination should be performed just prior to the start of irradiation.