Aims & Objectives: The burden and nature of adverse drug events (ADEs) in paediatric intensive care units (PICUs) are not well understood. This study aimed to assess the incidence, nature, preventability and severity of ADEs across three English PICUs. Methods: A prospective observational cohort study was carried out over a three-month period during 2019. PICU patients (≤18 years of age) who stayed for a minimum of 24-hours were included. Intensive surveillance for suspected ADEs was performed by trained clinical pharmacists. An expert panel assessed causality, preventability and severity of detected events. Results: Of 302 patients included, one or more ADEs was detected in 47 (15.6%) patients. A total of 62 ADEs were confirmed, with an estimated rate of 20.5/100 patients (95% CI, 15.3-27.5) and 16.7/1000 patient-days (95% CI, 9.3-29.9). The majority of ADEs were preventable (36/62, 58.1%). Most ADEs (42/62, 67.7%) caused temporary harm and associated with medicines prescribing (29/62, 46.8%). Medicines for the central nervous system (14/62, 22.6%) and infections (13/62, 20.9%) were commonly involved. Children with a hospital stay of 7 or more days (OR 5.28, 95% CI, 2.12-13.14) were more likely to experience an ADE compared to patients with a stay of 1-6 days. Rates of ADEs varied among the three PICUs (p<0.006). Conclusions: We report findings from the first epidemiological study to examine the frequency, nature, preventability and severity of ADEs in PICUs in the UK. Further study addressing underlying contributory factors of preventable ADEs is now needed to identify targets for remedial interventions and reduce the risk of avoidable patient harm.