The study aims to identify ‘missing’ diagnoses amongst paediatric admissions during the UK’s first national lockdown, compared with the previous 5 years. A retrospective observational cohort study of all children (0–15 years) attending for urgent care across Oxfordshire, during the first UK lockdown in 2020, compared to matched dates in 2015–2019, across two paediatric hospitals providing secondary care, including one with tertiary services. Our outcome measures were changes in numbers of patients attending and inpatient diagnoses (using ICD-10 classification) during the first 2020 lockdown, compared with the previous 5 years, were used. We found that total Emergency Department (ED) attendances ( n = 4030) and hospital admissions ( n = 1416) during the first UK lockdown were reduced by 56.8% and 59.4%, respectively, compared to 2015–2019 (5-year means n = 7446.8 and n = 2491.6, respectively). Proportions of patients admitted from ED and length of stay were similar across 2015–2020. ICD-10 diagnoses in lockdown of 2020 ( n = 2843) versus matched 2015–2019 dates ( n = 19,946) demonstrated significantly greater neoplasm diagnoses ( p = 0.0123). Of diagnoses ‘missing’ in lockdown, 80% were categorised as infectious diseases or their sequelae and 20% were non-specific pains/aches/malaise and accidental injury/poisonings. Conclusions : Pandemic public health measures significantly altered paediatric presentations. Oxfordshire hospitals had a 58% reduction in ED attendances/inpatient admissions, with ‘missing’ diagnoses predominantly infection-related illnesses. These are likely driven by a combination of the following: (1) public health infection control measures successfully reducing disease transmission, (2) parents/carers keeping mild/self-limiting disease at home, and (3) pandemic-related healthcare anxieties. Prospective studies are needed to ensure referral pathways identify vulnerable children, those with social concerns, and avoid delayed presentation. What is Known: • Significant reductions of paediatric ED attendances and inpatient admissions are reported globally, throughout national and regional lockdowns for COVID-19. • Previous studies (supplemental table 5 ) examined only ED diagnoses or specific inpatient diagnoses during lockdown periods, demonstrating reductions of infectious diseases, accidents/injuries, and safeguarding referrals. What is New: • Using ICD-10 coding, robustly controlling for five historical years and adopting a hypothesis-independent analysis, demonstrating 80% of ‘missing’ inpatient diagnoses during national COVID-19 lockdown were infectious dise...

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