Endomyocardial biopsies (EMB) of the left ventricle (LV) were performed during cardiac catheterization in 27 patients with apparently primary myocardiopathies (PMC). The results of optical and electron microscopy were compared with ventricular performance indices calculated from hemodynamic and cinematographic data from the left ventricle. The telediastolic pressure of the LV increases with the spread of fibrosis (r = 0.68) suggesting that it disturbs the filling of this cavity. Systolic effort is reduced as this fibrosis increases (r = 0.56), and it contributes to the changes in the pumping function of the LV, but the contractile power of the LV is altered in a less evident and less constant manner. The association of cell and fibro-elastic tissue degeneration is always accompanied by a drop in ventricular performance (p less than 0.0001). Apart from its diagnostic role, which is limited but may be of great value, the EMB can help to clarify the physiopathology of PMC and to formulate a prognosis based on more solid grounds.