Left ventricular myocardial function was studied in 13 patients with primary nonobstructive myocardiopathy and compared to the values determined in two control groups. Peak rate of rise of ventricular pressure (max dp/dt) and rate of the proportional rise of ventricular pressure at max dp/dt [(max dp/dt)p<sup>-1</sup>] were significantly decreased and interval t-max dp/dt was significantly prolonged in primary myocardiopathy, indicating that decreased contractility is characteristic of this disease. Left ventricular pre-ejection period (PEP) was significantly prolonged and PEP/LVET (left ventricular ejection time) ratio was significantly higher in patients with primary myocardiopathy. Increased end diastolic volume (EDV) and end diastolic pressure (EDP) were not characteristic of this disease. Best correlations were found between (max dp/dt)p<sup>-1</sup> and t-max dp/dt (r = -0.71). Good correlations between PEP/LVET and internal indices of myocardial contractility qualify this ratio as a simple and useful index of myocardial contractility in various heart diseases.