question: INTRODUCTION Royal College of Paediatrics and Child Health sub-specialist training in Paediatric Clinical Pharmacology and Therapeutics has been delivered in the UK for 20 years, but no specialist clinical services have been set up previously. METHODS Prospective audit and service evaluation of paediatric clinical pharmacology service pilot phase and dedicated service at a UK children's hospital. RESULTS Pilot scheme (May-Oct 2019), then weekly service (established June 2020). Service covers the High Dependency Unit, and inpatients with polypharmacy. The pilot demonstrated high levels of acceptance, with 89% of suggested medication changes agreed by lead clinical team, and success, with 97.5% of suggested changes continued until discharge/pilot completion. Economic analysis estimated direct annualised cost savings on medications of up to £10,000. After 20 ward rounds of the established service, 270 potential medication changes were identified, 213 were carried out (78.9%). The most common were deprescribing (n=143), prescribing (n=47) and dose adjustment (n=8). Seventy-five different medications were deprescribed, most commonly Chloral Hydrate (n=12), Lactulose, Ibuprofen, Bio-Kult, and Sodium alginate (all n=4). The percentage of inpatients prescribed ≥10 medications decreased from 38.5% to 32.1%, while the subset prescribed ≥20 medications decreased from 11.0% to 5.67%. The mean number of medicines prescribed decreased from 9.0 to 8.0, while the median was unchanged at 7. Annual Yellow card reports of suspected adverse drug reactions more than doubled (n=66). CONCLUSION A UK model for sub-specialist paediatric clinical pharmacology service delivery has demonstrated a positive clinical impact and could be replicated at other UK secondary/tertiary children's hospitals.