Aim: To develop and validate an algorithm to rapidly distinguish transient synovitis(TS) of the hip from differential diagnoses without additional tests.Methods: This retrospective cohort study included all children admitted for non-traumatic limping in the emergency department at Lille University-Hospital between2016 and 2020. The gold standard was a definitive diagnosis at follow-up visit. Allvariables associated with acute limping in children were analysed in univariate andmultivariable analyses. An algorithm was then developed using recursive partitioningand validated internally on a subset of patients.Results: There were 995 patients included (mean age 5.3 years; males 63%); 337had a TS including 210 confirmed at follow-up visit and 354 another diagnosis.After multivariable analysis, the relevant variables for distinguishing between TSand differential diagnoses were: age 3–10 years, absence of fever, absence of localinflammation, sudden onset of limping on awakening. An algorithm combining thesevariables was developed (n = 297) and validated internally (n = 175) for children>12 months with limping for ≤10 days, with a specificity of 98.2% and a positivelikelihood ratio of 19.6. No serious differential diagnoses were missed.Conclusion: Use of this algorithm enables the diagnosis of TS without additional testsand without missing serious differential diagnoses.