Aim: Paediatric acute liver failure (P-ALF) is a rare and devastating condition that leads to death or liver transplantation (LTx) in 40%–60% of cases. Determining the aetiology can enable disease-specific treatment, aid in prognostication for hepatic recovery and guide the decision-making for liver transplantation. This study aimed to retrospectively evaluate a systematic diagnostic approach to P-ALF in Denmark and to collect epidemiological nationwide data. Methods: All Danish children aged 0–16 years with P-ALF diagnosed between 2005 and 2018, and who were evaluated using a standardised diagnostic assessment programme, were eligible for retrospective analysis of clinical data. Results: A total of 102 children with P-ALF were included (presentation at 0 days to 16.6 years of age, 57 females). Aetiological diagnosis was established in 82% of cases, the remainder were indeterminate. Fifty percent of children with P-ALF of indeterminate aetiology died or underwent LTx within 6 months after their P-ALF diagnosis, compared to 24% of children with an aetiological diagnosis, p = 0.04. Conclusion: Following a systematic diagnostic evaluation programme, made it possible to identify the aetiology of P-ALF in 82% of cases which is associated with improved outcomes. The diagnostic workup should never be considered complete but rather adapt to ongoing diagnostic advances.