In Japan, cardiovascular diseases are frequent complications in cancer patients owing to the rapidly aging population and changes in the overall lifestyle. In addition, cancer itself including advanced cancer, and certain therapies used to treat it, often cause further cardiovascular complications. Moreover, although the recently developed molecularly targeted cancer drugs have improved patient prognosis, unexpected cardiovascular side effects, especially cardiotoxicity, have become an important limitation of cancer therapy. Recent molecularly targeted therapies for cancer may result in a variety of cardiovascular side effects, such as cardiac dysfunction, hypertension, and thromboembolism. As such, cardiologists should fully understand these molecularly targeted therapies and the potential pathogenic mechanisms at molecular levels. To protect cancer patients from cardiotoxicity, further basic and clinical research on the pathogenesis and mechanisms of cardiotoxicity of the molecularly targeted cancer drugs is of paramount importance. Furthermore, in-depth research in these areas could potentially result in new insights about onco-cardiology and cardiovascular medicine.