The existence of a new cascade, angiotensin-converting enzyme (ACE) 2/angiotensin (Ang)-(1-7)/Mas receptor axis, has been recently established in the renin-angiotensin system. However, the dynamics of this cascade under various pathological conditions in clinical settings is still unclear. Forty-nine patients who underwent emergency hospitalization because of acute heart failure (AHF) consented to participate in this study. Thirty-eight healthy volunteers served as controls. Serum ACE activity, ACE2, Ang-(1-7) concentration, plasma Ang II, aldosterone concentration, and plasma renin activity (PRA) were measured at the acute stage. We conducted a comparative study between patients with AHF and healthy volunteers. Patients with AHF showed lower serum ACE activity and plasma aldosterone concentration than healthy volunteers (12.3 vs. 15.1IU/L, respectively; P=0.01, 75.6 vs. 125.3pg/mL, respectively; P=0.000); there were no differences between the two groups in PRA and plasma Ang II concentration. Patients with AHF had a higher serum ACE2 concentration than healthy volunteers (7.9 vs. 4.8ng/mL, respectively; P=0.002), but their serum Ang-(1-7) concentration was significantly lower (2.4 vs 3.1ng/mL, respectively; P=0.005). Patients with AHF had a higher serum ACE2 concentration, lower serum Ang-(1-7) concentration, and lower serum ACE activity and plasma aldosterone concentrations than healthy volunteers, whereas PRA and plasma Ang II concentration were the same.