Eighty-five consecutive patients with idiopathic dilated cardiomyopathy were categorized according to the presence (biventricular dysfunction) or absence (left ventricular [LV] dysfunction) of reduced right ventricular ejection fraction (<35%) along with reduced LV ejection fraction (<50%). Compared with the 36 patients with LV dysfunction, the 49 patients with biventricular dysfunction had significantly worse New York Heart Association functional class (2.7+/-0.6 vs 1.9+/-0.5; p <0.001), LV ejection fraction (26+/-10% vs 34+/-8%; p <0.0001), and outcome (transplant-free survival, 55% vs 89%; p <0.001). Thus, dilated cardiomyopathy is frequently characterized by biventricular involvement, which identifies a more severe disease and a worse long-term prognosis.