The natural history of hypertrophic cardiomyopathy (HCM) is complex and may include progressive heart failure and severe left ventricular dysfunction. When disease progression is abrupt, however, other coexisting diseases should be ruled out. This may be difficult in the case of amyloidosis, which classically mimics HCM. We present an example of severe clinical deterioration in a patient with HCM due to superimposed amyloid light-chain amyloidosis. A man in his 70s with a longstanding history of genetically confirmed HCM presented with rapid development of congestive heart failure over 6 months, in sharp contrast to a previously stable, asymptomatic clinical course. He was diagnosed as having the illness in his late 40s after a resuscitated cardiac arrest and regularly followed up on a yearly basis. His most recent electrocardiogram was profoundly changed from previous tracings, with marked and diffuse voltage reduction (QS in V1-V3) and inferolateral T-wave inversion. The echocardiogram showed an abrupt increase in the severity of left ventricular (LV) hypertrophy, with a concentric rather than asymmetric appearance, granular sparkling of the myocardium, biatrial enlargement, thickening of the mitral valve leaflets, and interatrial septum and mild pericardial effusion. Severe LV dysfunction with a restrictive LV filling pattern was evident, which is associated with LV outflow tract obstruction loss and right ventricle systolic impairment. Following hospital admission, multiple myeloma was diagnosed and confirmed by bone marrow biopsy and aspiration. Furthermore, abdominal fat aspiration showed amyloid deposition and confirmed the diagnosis of amyloid light-chain amyloidosis. Electrocardiograms, echocardiographic images, and videos presented in this report describe the abrupt and marked evolution of a sarcomeric to infiltrative cardiomyopathy, leading to an ominous outcome in which the patient died despite specific treatment. While progression to the end-stage phase occurs over several years for patients with HCM and can be detected at relatively early stages, the abrupt onset of congestive heart failure is uncommon and should raise suspicion of other, superimposed cardiac diseases.