Summary: The number of cardiac β-adrenoceptors and the positive inotropic effect of isoprenaline and milrinone were measured in cardiac membranes and isolated, electrically driven muscle strips from nonfailing donor hearts and from patients with mitral valve disease (NYHA II-III), ischemic heart disease, and dilated cardiomyopathy (NYHA IV). In nonfailing hearts, the number of β-adrenoceptors were 41.5 fmol/mg protein (mean, n = 3). In ischemic heart disease and NYHA II III. there was a loss of cardiac β-adrenoceptors (22.1 fmol/mg protein. mean, n = 3; 23.2 ± 2.7 fmol/mg protein, n = 30). respectively. In NYHA IV. there was a pronounced reduction of the number of cardiac β-adrenoceptors to 12.1 ±1.5 fmol/mg protein (n = 15). The Kd value did not differ in either group. Correspondingly, the positive inotropic effect of isoprenaline was more pronounced in nonfailing myocardium, reduced in NYHA II III and ischemic heart disease and almost blunted in NYHA IV. Similar results were observed with the phosphodiesterase inhibitor milrinone. A good correlation of the β-adrenoceptor density to the maximal positive inotropic effect of isoprenaline and milrinone was observed. Neither the number of cardiac β-adrenoceptors nor the positive inotropic effect of isoprenaline correlated with the age of the patients. We conclude that the number of cardiac β-adrenoceptors and the positive inotropic effect of β-adrenoceptor agonists are reduced in the failing human heart depending on the severity of heart failure. Furthermore, the positive inotropic effect of milrinone is also reduced and related to the reduction of β-adrenoceptors. The lack of correlation with the age of the patients provides evidence for a predominant role of heart disease rather than aging in the reduction of β-adrenoceptors and subsensitivity to cyclic AMP-increasing positive inotropic agents in the failing human heart.