This investigation was designed to evaluate the difference in DNA activity between biopsy specimens obtained from right and left ventricles. Nucleic DNA in the myocardial cells of hypertrophied and congestive forms of cardiomyopathy was analyzed to investigate the relationship between cell function and clinical manifestations. Endomyocardial biopsy specimens were obtained simultaneously from right ventricular septal wall and left ventricular inferolateral wall by a transcatheter biotome. Measurement of DNA was based on the Feulgen reaction and dual wavelength cytophotometry. In this series, 12 patients with hypertrophic cardiomyopathy and four patients with congestive cardiomyopathy were studied. In the normal heart, the DNA value (arbitrary units) of the right ventricle was 138.6, whereas that of the left ventricle was 144.4. In the hypertrophic group, the mean DNA value in the right ventricle was 279.9, whereas that of the left ventricle was 317.5. In the congestive group, the DNA mean value in the right ventricle was 108.8, whereas that of left ventricle was 144.8. The linear relationship (r = 0.67) between right and left ventricular DNA values suggests that cellular function of one ventricle is affected by that of the other side. Higher DNA values of the left ventricle may indicate the difference in work load between the ventricles. The relationships among DNA values, LV wall thickness, LV mass, and parameters of contractility were statistically high.