Objectives: To determine whether the introduction of the capsular group B meningococcal vaccine (4CMenB) in the UK has increased presentations of infants to emergency departments with adverse events following immunisation (AEFI). Participants, design and setting: A retrospective review of hospital records of infants aged 1-6 months presenting to Oxford University Hospitals NHS Trust's emergency departments from September 2013 to August 2016 with discharge diagnoses of vaccine reactions or non-specific conditions. Immunisation history was checked by reference to centralised immunisation records. Main outcome measures: Presentation classifications were 'probable vaccine reaction' (ie, symptoms within 48 hours of immunisation; no alternative cause found), 'possible vaccine reaction' (symptoms within 48 hours of immunisation with a possible alternative cause) or 'not related' (clear alternative diagnosis or not immunised within previous 48 hours). Results: Prior to 4CMenB introduction (2013-15), an annual average of 12 infants presented with probable or possible AEFIs, increasing to 38 infants in the year following 4CMenB introduction (2015/2016). Rates of AEFIs per 1000 immunisation episodes increased post-4CMenB introduction from 1.03 to 3.4 (P<0.001) at 2 months and from 0.14 to 1.13 (p=0.005) at 4 months. At 3 months, when 4CMenB is not given, no increase was seen (p=0.380). 4CMenB introduction was also associated with increased AEFI-related hospital admissions, invasive investigations and intravenous antibiotic use. Conclusions: The increase in emergency department attendances, investigations and antibiotic use for AEFIs following 4CMenB immunisation may influence the cost-effectiveness of the 4CMenB immunisation campaign.