Background: Plasma renin activity (PRA) has gained relevance as prognostic marker in adults with heart failure. The use of PRA as a clinically meaningful parameter in children and children with heart failure requires a thorough knowledge of the factors that influence PRA to correctly assess PRA levels. We aim to evaluate the influence of age, heart failure and angiotensin-converting enzyme inhibitor (ACEi) on PRA levels in children. Methods: We conducted a systematic literature search to identify studies on PRA levels in healthy children and in children with heart failure. In addition, we analysed PRA data measured before (n = 35, aged 25 days–2.1 years), 4 hours after (n = 34) and within the first 8 days of enalapril treatment (n = 29) in children with heart failure from the European project Labeling of Enalapril from Neonates up to Adolescents (LENA). Results: Age has a profound effect on PRA levels in healthy children, as PRA levels in the literature are up to about 7 times higher in neonates than in older children. Children with heart failure younger than 6 months showed 3–4 times higher PRA levels than healthy peers in both the literature and the LENA studies. In the LENA studies, the ACEi enalapril significantly increased median predose PRA by a factor of 4.5 in children with heart failure after 4.7 \pm 1.6 days of treatment (n = 29, p 0.01). Prior to treatment with enalapril, LENA subjects with symptomatic heart failure (Ross score \ge3) had a significantly higher PRA than LENA subjects with asymptomatic heart failure of comparable age (Ross score \le2, p 0.05). Conclusions: Age, heart failure and ACEi treatment have a notable influence on PRA and must be considered when assessing PRA as a clinically meaningful parameter. Clinical Trial Registration: The trials are registered on the EU Clinical Trials Register (https://www.clinicaltrialsregister.eu). Trial registration numbers: EudraCT 2015-002335-17, EudraCT 2015-002396-18.