Left ventricular end-diastolic pressure (P) and volume (V) were measured in 12 patients with acute myocardial infarction. The diastolic P-V relationship was apparently exponential and corresponded to the formula P = beKV. In 7 patients submitted to volume loading, several data points of this relationship were obtained and at zero volume, the mean intercept with the ordinates was 0.037 .+-. 0.015 kPa [kilopascal] (SEM [standard error of the mean]) (0.28 .+-. 0.12 mmHg). In the other 5 patients, the P-V curve was plotted through this intercept and the pressure and volume co-ordinates obtained by control. The K coefficient (passive elastic modulus) was greater, and the normalized left ventricular compliance index (dV/VdP) was smaller in the infarct group than in the control group. This suggests decreased left ventricular compliance during the acute phase of myocardial infarction. By comparing left ventricular function curves plotted using either end-diastolic pressure or end-diastolic volume as the stretch index it is possible to evaluate the relative participation of decreased compliance and depressed contractility in global left ventricular function.