- Background: Delayed antibiotic treatment for acute otitis media (AOM) is recommended for children >6 months with nonsevere illness, no risk factors for complications or history of recurrent AOM. This study evaluates relationship between delayed antibiotic treatment for antecedent AOM and severity of subsequent acute mastoiditis admission. Methods: A prospective observational study of children aged 0–14 years admitted with acute mastoiditis to 8 hospitals between 2007 and 2012 calculates rates of severe acute mastoiditis admission [defined by ≥1 of the following: complication (mastoid subperiosteal abscess, brain abscess and sagittal vein thrombosis), need for surgical procedure and duration of admission >6 days].Severe acute mastoiditis admissions in children with antecedent AOM treated with immediate antibiotics were compared with those with delayed antibiotic treatment. Results: Antecedent AOM was diagnosed in 216 of 512 acute mastoiditis admissions (42.1%), of whom 159 (73%) immediately received antibiotics, and 57 (27%) had delayed antibiotic treatment. Higher rate of recurrent AOM was noted in the immediate compared with delayed antibiotic treatment group (29% vs. 8.7%, P = 0.0021). Complication rates were 19.5% versus 10.5% (P = 0.12), rates of surgical procedures required, 30% versus 10% (P = 0.0033); admission rates >6 days, 37% versus 29% (P = 0.28) for immediate antibiotic therapy and delayed antibiotic treatment. On logistic regression analysis, immediately treated AOM patients had increased need for surgery for acute mastoiditis with adjustment for history of recurrent AOM (relative risk: 3.2, 95% confidence interval: 1.4–7.0). Conclusions: Delayed antibiotic treatment for antecedent AOM is not associated with an increase in severity parameters in subsequent acute mastoiditis admission.