The heart depends critically on continuous blood supply, but it is unknown whether cancer itself affects myocardial blood flow (MBF). This study investigated MBF in cancer patients and cardiac morphology in a cancer mice model. MBF was quantified with [¹⁵O]H₂O positron emission tomography at rest in recently diagnosed breast cancer patients and age-matched female controls, and additionally during 10-min exercise in the cancer patients. Cardiac morphological changes were analyzed with a breast cancer mouse model and control mice without tumors. Resting MBF was similar in cancer patients and controls. MBF increased significantly during exercise in cancer patients, and exercising MBF correlated positively with cancer grade. In the mouse model, cancer did not affect heart weight, cardiomyocyte size, myocardial capillary density, or capillary-to-myocyte size ratio. Thus, resting MBF in humans or myocardial capillarity in mice appears not to be affected by breast cancer. The exercise-induced MBF increase in cancer patients with higher histologic grade requires further investigations.