This report describes very rare siblings who had a left ventricular muscular diverticulum and hypertrophic cardiomyopathy. The first case is a 16-year-old male. On two-dimensional echocardiography, a left ventricular muscular diverticulum in the posterior wall and mitral valve prolapse were detected. The former was verified by left ventriculography. Endomyocardial biopsy showed findings compatible with hypertrophic cardiomyopathy. The second case is a 13-year-old female. Two-dimensional echocardiography revealed a left ventricular muscular diverticulum in the same location as that of the first case, and mitral valve prolapse as well. The former was confirmed by left ventriculography. The endomyocardial biopsy findings were compatible with hypertrophic cardiomyopathy. The coronary angiograms were normal in both cases. To our knowledge, familial appearance of a left ventricular diverticulum has not yet been reported, and a left ventricular diverticulum at the posterior wall in cases with hypertrophic cardiomyopathy is very rare.