Hypertrophic cardiomyopathies (HCM) are a frequent cause of sudden death in both young people and adults. Different cardiomyopathies can be distinguished according to the etiological agent and, although there are morphological differences too, alterations in the quantity of DNA in the cardiomyocytes may play an important role in their pathogenesis and evolution. To understand the characteristics and the behaviour of the DNA index in hypertrophic cardiomyopathies, we have studied thirty cases (10 primaries or essential, 10 hypertensives and 10 toxic) and compared the results with those obtained for 10 macroscopically normal hearts. The results showed that the different cardiomyopathies were statistically related with age, heart weight and ventricle thickness. The normal hearts showing a diploid DNA index, hypertensive cardiomyopathies (CM) hearts a tetraploid index and both toxic CM (1.1-1.9) and essential CM (>2) heart showing an aneuploid index. Statistically significant associations (p < 0.001) were observed when the DNA values were compared with the type of CM, the age, the thickness of both ventricles and the heart weight. Therefore, we think that the technique described may be of great help in the differential diagnosis of hypertrophic cardiomyopathies.