A method using a calibrated ruler for measuring lymph node depth on lateral lymphoscintigrams is described. The accuracy of the method was confirmed with a plexiglass phantom. At the time of internal mammary lymphoscintigraphy, lateral images were obtained in 201 patients. A total of 545 lymph nodes were visualised and categorised into 13 depth intervals. The average lymph node depth was 3.0 ± 1.1 cm with a range of 1.0–7.1 cm. Average lymph node depth was classified according to body weight with an average depth of 2.5, 3.1 and 3.7 cm for patients weighing less than 60, 60–75 and more than 75 kg, respectively. Lateral lymphoscintigrams demonstrated normal anatomical communication between internal mammary and anterior mediastinal lymph nodes in 3.5% of patients.