Two siblings with hypertrophic cardiomyopathy developed spontaneous complete heart block requiring permanent pacemaker implantation at similar ages (29 and 33 years). The clinical, morphological, and haemodynamic expressions of hypertrophic cardiomyopathy differed considerably in these two patients. The sister had severe functional limitation due to dyspnoea, pronounced and diffuse left ventricular hypertrophy (maximum ventricular septal thickness of 41 mm), and left ventricular outflow obstruction (peak subaortic gradient of 75 mm Hg under basal conditions). In contrast the brother was symptom free, had only modest left ventricular hypertrophy which was confined to the anterior ventricular septum (maximal thickness of 16 mm), and had no echocardiographic evidence of subaortic obstruction. These dissimilar findings in siblings with hypertrophic cardiomyopathy suggest that the predisposition to develop complete heart block was probably genetically transmitted, although it was unrelated to the phenotypic and clinical expression of the disease.