The construction of a Frank-Starling myocardial function curve relating heart work to left ventricular preload is clinically utilized to assess therapeutic protocols in critically ill patients. The pulmonary capillary wedge pressure (PWP) is the index of left ventricular filling pressure most frequently utilized as representative of left ventricular preload.The authors assessed the relationship between left ventricular preload measured as the left ventricular end-diastolic volume (LVEDV), and the PWP, in acutely ill patients with sepsis and cardiac disease. Within each group, no relationship was found between the LVEDV and the PWP; however, when omitting the effect of PEEP, a modest correlation was noted (r= 0.302;p< 0.01). Of the left ventricular ejection fraction. LVEDV and PWP, the PWP accounted for less than 5% of the explained variance in the stroke volume index.The PWP is a poor predictor of left ventricular preload, probably because of abnormalities of left ventricular compliance in critically ill patients.