OBJECTIVE: To better understand testing patterns in children, we measured temporal trends in paediatric testing from 2005 to 2019 in Oxfordshire, UK. DESIGN: Descriptive study of population-based secondary data. SETTING: Oxfordshire University Hospitals National Health Service Trust laboratories. PARTICIPANTS: Children aged 0-15 years in Oxfordshire who received at least one blood test. MAIN OUTCOME MEASURES: We estimated average annual percentage changes (AAPCs) in test use using joinpoint regression models. Temporal changes in age-adjusted rates in test use were calculated overall and stratified by healthcare setting, sex, and age. RESULTS: Between 2005 and 2019, 1 749 425 tests were performed among 113 607 children. Overall test use declined until 2012, when test rates appeared to increase (AAPC 1.5%, 95% CI -0.8% to 3.9%). Most tests were performed in inpatient settings, where testing rates stayed steady (AAPC -0.6%, 95% CI -2.1% to 0.9%). Increases were highest in females, those aged 6-15 years and in the outpatient setting. The greatest increase in testing was for vitamin D (AAPC 26.5%), followed by parathyroid hormone (9.8%), iron studies (9.3%), folate (8.4%), vitamin B12 (8.4%), HbA1c (8.0%), IgA (7.9%) and coeliac (7.7%). CONCLUSIONS: After an initial decline, laboratory test use by children in Oxfordshire demonstrated an apparent increase since 2012. Test use increased in outpatient and general practice settings, however remained steady in inpatient settings. Further research should examine the root causes and implications for test increases, and whether these increases are warranted. We encourage clinicians to consider the individual and systemic implications of performing blood tests in children.