To analyse bilateral differences in the cross-sectional area of the internal mammary artery (IMA) and vein (IMV) in breast cancer patients compared to healthy controls.On 135 breast MRIs the cross-sectional areas of the IMA and IMV were measured on the left and right side in the second and third intercostal space (ICS) by two independent readers. Differences were analysed using a linear mixed model.In the healthy control group (n = 91) no significant differences between the cross-sectional areas of the IMA and IMV were observed. Both readers reported a mean adjusted difference of 0.12?mm2 (p = 0.298) and 0.21?mm2 (p = 0.058) for the IMA in the second ICS. In the malignancy group (n = 44) the cross-sectional area was significantly larger on the malignancy side compared to the contralateral side. The largest difference in the IMA was measured in the second ICS with a mean adjusted difference for reader 1 of 1.37?mm2 (p <0.001) and for reader 2 of 0.81?mm2 (p = 0.003).The vascular cross-sectional area of internal mammary vessels was significantly different on the side with breast cancer compared to the contralateral side. This difference was not observed in healthy controls.? MRI has become an important imaging modality in the diagnostic workup of breast cancer. ? In healthy persons no significant difference in the size of the left and right IMA is observed. ? A significant enlargement of the IMA on the malignant side occurs in most patients.