The radionuclide angiograms of 59 patients with a left ventricular (LV) ejection fraction (EF) less than 0.40, 23 with idiopathic dilated cardiomyopathy (IDC) and 36 with coronary artery disease (CAD) were analyzed to assess the usefulness of radionuclide angiography in distinguishing these conditions. Mean right ventricular EF was lower in the IDC group than in the CAD group, 0.31 vs 0.45 (p less than 0.01). LV wall motion was scored from 3 (normal) to -1 (dyskinesia). The incidence of akinesia was similar in IDC and CAD groups, 70% and 83%, respectively. Dyskinesia was more common in the CAD group (42% vs 17%), but the difference was not statistically significant. Segmental wall motion analysis showed similar patterns of wall motion in both groups, with contraction best preserved in the anterobasal, posterobasal and superolateral segments. Patients in the CAD group had worse apical motion (p less than 0.01) and better wall motion in the anterobasal (p less than 0.05) and superolateral walls (p less than 0.01), compared with patients in the IDC group. To assess symmetry of contraction, a maximum difference score was derived for each patient. Symmetry (a score less than 1) was present in 5 IDC and no CAD patients, whereas asymmetry (a score of 2 or more) was present in 27 CAD and 7 IDC patients (p less than 0.01). Wall motion became more symmetric in both groups when LVEF was less than 0.20. Logistic regression analysis revealed that the maximum difference score was the best predictor of the diagnosis, but only because of better separation at the extremes of maximum difference score values.(ABSTRACT TRUNCATED AT 250 WORDS)