Physicians have been urged to reduce the use of the pulmonary artery catheter. However, there are no guidelines to help the clinician make the decision to use or withhold invasive monitoring in the individual patient. This study was designed to examine the accuracy of physician estimates of cardiac function in a spectrum of patients with hemodynamic instability to determine whether differences in accuracy among subgroups would suggest subgroups of patients who could be managed without invasive measurements. Physician estimates of cardiac index were found to be sufficiently accurate in patients without acute heart disease that initial management without invasive monitoring may be appropriate in selected cases. However, due to the general inaccuracy of physician estimates, efforts to improve the accuracy of clinical judgments of cardiac function and hemodynamic status should be pursued with vigor in patients both with and without acute cardiac dysfunction.