To evaluate left ventricular mechanical energy efficiency and ventriculo-arterial coupling in humans, left ventricular pressure-volume relations were determined using the conductance catheter technique in 20 patients undergoing cardiac catheterization. The results were as follows: 1. A convex, curvilinear relationship was observed between end-systolic pressure-volume relations (Emax) and left ventricular ejection fraction (EF), as shown in the equation of EF = 28.5 x log (Emax) + 39.6 (r = 0.67, p less than 0.01, n = 20); EF remained nearly constant in the range of Emax greater than or equal to 4 mmHg/ml/m2, whereas, EF decreased markedly under the Emax of 4 mmHg/ml/m2. 2. A convex, curvilinear relationship was observed between Emax and mechanical energy efficiency (EW/PVA), as shown in the equation of EW/PVA = 30.5 x log (Emax) + 54.8 (r = 0.83, p less than 0.01, n = 16). 3. A concave, curvilinear relationship was observed between Emax and ventriculo-arterial coupling (Ea/Emax), as shown in the equation of Ea/Emax = -0.8 x (Emax)0.5 + 2.6 (r = -0.85, p less than 0.01, n = 16). Accordingly, EW/PVA was markedly decreased with changes in Emax in the range less than 4 mmHg/ml/m2, although it remained slightly increased above 4 mmHg/ml/m2. Ea/Emax was maintained constant (p less than 0.5) in the range of Emax above 4 mmHg/ml/m2 but was abruptly decreased when Emax was reduced below 4 mmHg/ml/m2. These results indicate that depressed left ventricle attempts to work effectively at the risk of mechanical energy efficiency and with suitable matching by aortic property. Application of pressure-volume relationships provides a new framework for evaluation and treatment of the failing heart.