Aims We aim to assess the reproducibility of QRS fragmentation (fQRS) on a multi‐centre dataset of patients with acute myocarditis (AM), including a histopathological validation in a subgroup with biopsy‐proven disease. Electrocardiogram (ECG) in patients with myocarditis is usually considered aspecific. ST changes and conduction anomalies have been commonly reported so far. We have previously described fQRS in patients with AM. Methods and results Patients admitted between 2008 and 2019 in two centres with a diagnosis of AM were included. Standard ECG, echocardiography, and cardiac magnetic resonance (CMR) findings were recorded at baseline and at follow‐up (FU). Eighty patients were analysed, 66 men (82%), with median age of 34 (26–43) years. Twenty‐two patients had biopsy‐proven AM. At presentation, 61 patients (76%) displayed fQRS. Median ejection fraction (EF) was 55% (43–60). Seventy‐two patients (90%) underwent CMR and displayed late gadolinium enhancement (LGE). ECG leads showed that fQRS correlated with distribution of LGE. In patients with positive biopsy, fQRS was present in 18 (81%). Median FU was 419 days (224–956). Complete FU was available for 64 patients (80%), and 33 patients (52%) displayed persistence of fQRS. Median EF was 60% (57–64). Eleven patients underwent a repeated biopsy at FU, eight of whom had persistent inflammation and fQRS. Fifteen patients (23%) had ventricular tachycardia, 14 of whom still showed fQRS. Conclusions In this cohort fQRS was confirmed as an additional useful ECG sign. Persistence of fQRS was associated with ongoing inflammation and with a poorer outcome in terms of ventricular function and occurrence of arrhythmias.