Angiotensin-I converting enzyme inhibitors (ACE-Is) are commonly used as safe antihypertensive agents, and it has recently been suggested that they decrease the risk of cancer development. Recent studies have revealed that the renin-angiotensin system (RAS) is involved in the development of many types of tumor. Angiotensin-II (AT-II) has many biological effects, including neo-vascularization, which plays a pivotal role in tumor development. AT-II induces a potent angiogenic factor, namely the vascular endothelial growth factor (VEGF). Some studies have proven that several ACE-Is are potent inhibitors of experimental tumor development and angiogenesis at clinically comparable doses. VEGF expression in tumors is also significantly suppressed by ACE-Is. When used in combination with the conventional anti-cancer drugs, ACEIs exert more potent anti-tumor activities as compared with either single agent, in addition to suppression of the intra-tumoral angiogenesis. Furthermore, ACE-Is reportedly not only suppress tumor growth but also attenuate the carcinogenesis process in which angiogenesis is involved. Since ACE-Is are already in widespread clinical use without any serious adverse effects, they may represent a potential new strategy for cancer therapy and chemoprevention.