Left ventricular myocardial stiffness was calculated in clinical cases. Thirty patients who underwent diagnostic cardiac catheterization were studied. Left ventricular cineangiograms and simultaneously recorded left ventricular pressure tracings were analyzed. The left ventricular stiffness constant k, was computed by substituting the left ventricular myocardial stress (sigma) and strain (epsilon) throughout the period between the end of rapid ventricular filling and the beginning of atrial contraction, in the equation sigma = b.ek.epsilon. Left ventricular myocardial stiffness was measured as follows: Group I (normal): 9.6 +/- 4.3, Group II (mitral stenosis): 10.7 +/- 4.0, Group III (left ventricular volume overload): 12.1 +/- 4.2, Group IV (coronary artery disease without myocardial infarction): 8.6 +/- 2.5, and Group V (myocardial infarction): 23.6 +/- 7.4. All of the normal cases showed stiffness constants of less than 15 and maxVcf of more than 2.0 circ/sec, and all the patients with histories of heart failure in Groups III to V showed stiffness constants of more than 15 and maxVcf of less than 1.0 circ/sec. For the rest of the patients, measurements of the stiffness constant were valuable for the assessment of patients' clinical status, especially when combined with contractility.