Efficiency is defined as the ratio of the energy delivered by a system to the energy supplied to it. Depending on the particular question being addressed, there exist a plethora of definitions of efficiency in medical texts, thus hampering their comparison. If only the ventricular work seen by the arterial system is under investigation, pressure-volume work will serve as a useful numerator. If, on the other hand, external and internal work together, i.e. the total mechanical work, is of interest, the pressure-volume area might be employed. Total myocardial oxygen consumption (MVO2) will be a useful denominator in the case of aerobic energy production. The MVO2 for the unloaded contraction must be assessed if, as in other energy transfer systems, net efficiency is to be addressed. If even smaller steps in the chain of energy transfer are to be investigated MVO2 for the arrested heart must be assessed. With an appropriate therapy, hemodynamic determinants can be varied, to improve cardiac efficiency. Nonetheless, measurement of all variables necessary for the calculation of efficiency remains a challenge, in particular in the clinical setting. Separation of the direct effects of drugs on efficiency is even more difficult, since hemodynamic conditions can hardly be controlled throughout the observation period, and changes in efficiency might be secondary to changes in hemodynamics. Whether the heart by itself employs mechanisms to improve its efficiency is still a matter of discussion: there is evidence that when oxygen supply decreases, the heart can switch from one substrate to a less costly one, or possibly can improve efficiency through better use of oxygen. Moreover, the heart seems to "sense" an even more decreased oxygen supply and reduce function in response. Myocardial stunning could be regarded as a protective mechanism as well, with function remaining depressed and the oxygen supply being normal or close to normal. One may conclude from the decreased efficiency that the excess oxygen consumption is used up for repair processes. The improved efficiency found in hypertrophied hearts represents another adaptive process. The underlying mechanism is unclear: a shift towards isomyosin V3 or some undefined shift in metabolic pathway is discussed. It is also still a moot question towards which objective the efficiency of the heart is adjusted. It has been described that under physiologic conditions, the efficiency of both the left and the right ventricle ought to be maximized.(ABSTRACT TRUNCATED AT 400 WORDS)