The differentiation of primary dilated cardiomyopathy from ischemic cardiomyopathy, though important, is difficult clinically and may require coronary angiography or metabolic imaging. Both patient groups have severe left ventricular dysfunction and severe wall motion abnormality. This study examined the differences in right ventricular performance between the two groups. There were 90 patients with a left ventricular ejection fraction less than 30% who had coronary angiography and multigated radionuclide angiography (MUGA). Of these, 69 had ischemic cardiomyopathy and 21 had primary cardiomyopathy. The left ventricular ejection fraction was similar; 22 ± 6% in ischemic cardiomyopathy and 21 ± 6% in primary cardiomyopathy. However, the right ventricular ejection fraction was higher in ischemic cardiomyopathy (38 ± 16% versus 29 ± 12%, p < 0.01). There were 59 patients with right ventricular ejection fraction ≥ 30%, of whom 50 patients (85%) had ischemic cardiomyopathy. The left ventricular and right ventricular volumes were determined by a count-based method. The right ventricular end-diastolic volume/left ventricular end-diastolic volume ratio was 0.57 in ischemic cardiomyopathy and 1.07 in primary cardiomyopathy (p < 0.05). Thus assessment of right ventricular function may help differentiate primary from ischemic cardiomyopathy; a preserved right ventricular performance is highly suggestive of ischemic cardiomyopathy. © 1992.