A 67-year-old woman was referred to our hospital with a sudden syncopal attack. She suffered from cardiogenic shock due to left ventricular (LV) outflow stenosis with simultaneous complete atrioventricular (AV) block. An endomyocardial biopsy of the left ventricle demonstrated myocardial disarray and myocardial fibrous and edematous tissue with infiltration of mononuclear cells. Cardiac magnetic resonance imaging (cMRI) detected a damaged septal area that was likely associated with the conduction disturbance. The diagnosis was hypertrophic cardiomyopathy accompanied by acute myocarditis. Although the LV outflow stenosis was transient, the complete AV block was persistent, thus requiring permanent pacemaker implantation.