Angiogenesis, the formation of new capillaries, is of central importance of physiological and pathophysiological processes, ranging from embryogenesis to wound healing, tumor growth and metastasis, and collateral circulation in atherosclerotic obstructive diseases. During exercise, capillary vessels in skeletal muscles are the major source of oxygen and nutrients. Exercise is also known to promote the infiltration of inflammatory cells like neutrophils into injured skeletal muscles. The aim of the present study was to investigate the role of oxidative products generated by phagocytes on capillary growth, using an in vitro angiogenesis system. The model system consisted of bovine capillary endothelial cells (BCEC) and rat smooth muscle cells. First, we confirmed that activated phagocytes converted L-tyrosine to the reactive aldehyde, pHA, through the myeloperoxidase (MPO)/peroxide/chloride system. Next, we examined the effect of pHA on angiogenesis. pHA dramatically inhibited the formation of capillary network. Moreover, pHA exerted cytotoxic effects on BCEC. These observations suggest that reactive aldehydes generated by activated phagocytes may play a critical role in regulating capillary growth at sites of inflammation.