Ultrastructural and biochemical alterations in myofibrils (Mf) were studied in biopsied myocardial tissues in 11 patients with dilated cardiomyopathy (DCM) and compared with those in non-hypertrophic control and secondary hypertrophic (SHT) heart muscles. Transverse diameters of biopsied cardiac myocytes increased significantly in both of SHT and DCM, and ultrastructural changes were similar in quality in both of them. Volume densities of Mf were 61.1 and 59.9% on average in control and SHT myocardial cells, respectively, and they were not significantly different. But in DCM volume density was significantly less (49.8% in left ventricular myocytes), and inverse relation between that and diameter of cardiac myocytes was observed (p less than 0.01). Electrophoretic pattern and relative composition of major structural proteins from control and SHT heart muscles were similar and statistically insignificant. In DCM, relative contents of myosin heavy chain and alpha-actinin decreased significantly and distinctively in all of cases suggesting primary degradation of myofibrils.