From the structural point of view ACE2 is closely related to ACE, and its discovery was a by-product of ACE investigations. However, this macromolecule has soon showed peculiar functional properties and has attracted increasing attention. The expression of ACE2 in the heart, kidney and vascular system pointed to a role in the regulation of blood pressure and cardiovascular homeostasis, possibly as a counterbalance to ACE activation, but this hypothesis now appears to be too simplistic. Additional functions of ACE2 are strongly suggested by its capability to modulate cytokine metabolism and the inflammatory reaction, as well as by its remarkable expression in the respiratory tract.