<b><i>Background/Aims:</i></b> Angiotensin-converting enzyme 2 (ACE2) is the only known active homologue of ACE, and degrades angiotensin (Ang) II and Ang I to Ang(1–7) and Ang(1–9), respectively. The role of ACE2 in kidney transplant (KT) is unknown. Our objective was to investigate circulating ACE2 activity in KT patients, and the relationship between serum ACE2 activity and age, gender, graft function and cardiovascular risk markers in KT patients. <b><i>Methods:</i></b> 113 KT patients with stable graft function were included in this cross-sectional study. Circulating ACE2 activity was assessed using a fluorescent assay. <b><i>Results:</i></b> Circulating ACE2 activity was detectable in KT patients and was increased in KT with ischemic heart disease as compared to KT without ischemic heart disease (105.9 ± 8.7 vs. 97.1 ± 7.05 relative fluorescence units (RFU)/µl/h, p < 0.05). ACE2 activity was increased in male KT as compared to females (105.2 ± 9.1 vs. 84.7 ± 6.9 RFU/µl/h, p = 0.05). ACE2 activity correlated positively with serum creatinine (r = 0.27), serum urea (r = 0.29), age (r = 0.24), aspartate transaminase (r = 0.39), alanine transaminase (r = 0.48), γ-glutamyl transferase (γ-GT) (r = 0.52), age (r = 0.24), and glycosylated hemoglobin (r = 0.19) (p < 0.05). By multiple regression analysis, age, serum creatinine, and serum γ-GT were independent predictors of serum ACE2 activity (r = 0.66, p < 0.001). <b><i>Conclusions:</i></b> Circulating ACE2 activity is measurable in KT patients and directly correlates with age, renal allograft and liver function parameters. These findings suggest that measurement of serum ACE2 may be used as a non-invasive marker to understand the role of the renin-angiotensin system in KT patients.