The acute administration of the angiotensin-converting enzyme (ACE) inhibitor captopril to healthy subjects transiently increases 5.5-fold the plasma levels of a natural stem-cell regulator, N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP). The aim of this study was to measure plasma Ac-SDKP levels during chronic treatment with all types of ACE inhibitors and to assess its relevance as a marker of ACE inhibition. Plasma levels of Ac-SDKP were blindly determined in age- and sex-matched hypertensive patients either treated (ACEI group, n=27) or not (non-ACEI group, n=23) with an ACE inhibitor for more than 1 month. Geometric mean [range] of plasma Ac-SDKP levels were significantly higher in the ACEI group (3.78 [1.48 to 14.5] pmol/mL) than in the non-ACEI group, with no overlap between the groups (0.75 [0.36 to 1.22] pmol/mL, P<.0001). The measurement of Ac-SDKP in plasma discriminated all the patients of the ACEI group, whereas the simultaneous determination of either in vitro (using hippuryl-histidine-leucine as substrate) or in vivo (angiotensin II/angiotensin I ratio) ACE activity failed to identify nine and five cases, respectively. We conclude that Ac-SDKP accumulates in plasma during chronic ACE inhibitor treatment. The long-term consequences of Ac-SDKP accumulation are unknown. The reliability of plasma Ac-SDKP measurement makes it the best marker of chronic ACE inhibition, which can help to verify patients' compliance to ACE inhibitor treatment.