Prognosis of acute myocarditis is generally benign, but fulminant cases exist which require advanced life support devices, such as percutaneous cardio-pulmonary support (PCPS) and ventricular assist devices (VAD), and lead to fatal outcomes. The purpose of this study was to identify predictors and their values at admission which might foreshadow a fulminant course of myocarditis. Data from 138 patients (mean age 42.0 years, 79 males) with a diagnosis of acute myocarditis in the Tokyo CCU Network database from 2007 to 2009 were analyzed retrospectively. Patients were divided into fulminant (in-hospital death, or PCPS or VAD requirement, N = 42) and non-fulminant groups (N = 96). Clinical data at admission were compared between them. Overall in-hospital mortality was 14.5%. On multivariate analysis, low systolic blood pressure (BPsys, odds ratio (OR)/mmHg 0.97; 95% confidence interval (CI) 0.93–1.00, p = 0.032) and electrocardiographic QRS complex prolongation (OR/10 ms 1.28; 95% CI 1.10–1.59, p = 0.0034) at admission were independent factors associated with a fulminant course. By receiver operator characteristic curve analysis, the area under the curve predicting a fulminant course was 0.769 for low BPsys and 0.821 for prolongation of QRS duration. The optimal cut-off value was 101 mmHg for BPsys (sensitivity 79.5%, specificity 68.0%), and 120 ms for QRS duration (sensitivity 72.2%, specificity 88.0%). Systolic hypotension and prolonged QRS on admission are predictors of a fulminant course of myocarditis.