The clinical and echocardiographic features of cardiac amyloidosis may closely resemble those of hypertrophic cardiomyopathy, and the disorders may thus be mixed up. The present study was undertaken in an attempt to identify features separating the two conditions by analysis of electro- and echocardiographic findings in patients with familial amyloid polyneuropathy and hypertrophic cardiomyopathy. Twenty-nine patients with familial amyloidosis and 22 with hypertrophic cardiomyopathy were studied. Particular attention was given to the sum of the S wave in V1 and R wave in V5 or V6, the echocardiographic left ventricular mass and cross-sectional area, the presence or absence of asymmetrical septal thickening, granular and sparkling myocardial appearance, thickened heart valves, systolic anterior motion of the mitral valve, and pericardial effusion. A granular and sparkling appearance of the myocardium and thickened heart valves were found to be the best predictors of cardiac amyloidosis, while low QRS amplitudes in relation to echocardiographic left ventricular mass and a pericardial effusion seemed less important. The presence of systolic anterior movement of the mitral valve, a large left ventricular mass and a sum of S in V1 and R in V5 or V6 greater than 35 mm indicated hypertrophic cardiomyopathy. When the four strongest predictors (left ventricular mass, thickened heart valves, a granular sparkling myocardial appearance, and systolic anterior movement of the mitral valve) were used to reclassify the present patients, 28 of 29 amyloidosis patients and 21 of 22 patients with hypertrophic cardiomyopathy were correctly categorized. Noninvasive methods may thus be useful for detecting the myocardial infiltrative process, and cardiac amyloidosis may be confidently diagnosed by typical noninvasive findings together with histopathological documentation of amyloid in an organ other than the heart.