BackgroundSepsis-induced endothelial dysfunction is proposed to cause angiotensin-converting enzyme (ACE) dysfunction and renin–angiotensin–aldosterone system (RAAS) derangement, exacerbating vasodilatory shock and acute kidney injury (AKI). Few studies test this hypothesis directly, including none in children. We measured serum ACE concentrations and activity, and assessed their association with adverse kidney outcomes in pediatric septic shock.MethodsA pilot study of 72 subjects aged 1 week–18 years from an existing multicenter, observational study. Serum ACE concentrations and activity were measured on Day 1; renin + prorenin concentrations were available from a previous study. The associations between individual RAAS components and a composite outcome (Day 1–7 severe persistent AKI, kidney replacement therapy use, or mortality) were assessed.Results50/72 subjects (69%) had undetectable ACE activity (< 2.41 U/L) on Day 1 and 27/72 (38%) developed the composite outcome. Subjects with undetectable ACE activity had higher Day 1 renin + prorenin compared to those with activity (4533 vs. 2227 pg/ml,p= 0.017); ACE concentrations were no different between groups. Children with the composite outcome more commonly had undetectable ACE activity (85% vs. 65%,p= 0.025), and had higher Day 1 renin + prorenin (16,774 pg/ml vs. 3037 pg/ml,p< 0.001) and ACE concentrations (149 vs. 96 pg/ml,p= 0.019). On multivariable regression, increasing ACE concentrations (aOR 1.01, 95%CI 1.002–1.03,p= 0.015) and undetectable ACE activity (aOR 6.6, 95%CI 1.2–36.1,p= 0.031) retained associations with the composite outcome.ConclusionsACE activity is diminished in pediatric septic shock, appears uncoupled from ACE concentrations, and is associated with adverse kidney outcomes. Further study is needed to validate these findings in larger cohorts.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13054-023-04518-2.