BACKGROUND: Monitoring of pulmonary artery pressure is an essential component of the care of critically ill patients. The conditions under which reliable measurements can be obtained must be clarified. OBJECTIVES: To determine (1) whether reliable measurements of pulmonary artery pressure can be obtained with patients in the right or left 60 degrees lateral position and (2) which characteristics of patients preclude obtaining reliable measurements. METHODS: One hundred five patients (65 cardiac surgery, 40 general medicine) with pulmonary artery catheters were enrolled in a prospective, stratified, quasi-experimental study. Subjects were repositioned from supine (head of bed elevated < 30 degrees with 1 pillow) to the left and right 60 degrees lateral positions. Systolic, diastolic, and mean pulmonary artery pressures and pulmonary capillary wedge pressure were measured before and 5, 10, and 20 minutes after lateral repositioning. The zero reference was the phlebostatic axis when patients were supine and the dependent midclavicular line at the level of the fourth intercostal space when patients were in the lateral positions. RESULTS: In most patients, measurements obtained with patients in the lateral position differed significantly from measurements obtained with patients supine. None of the variables examined were reliable predictors of which patients would have these differences. More than 11% of the patients had clinically significant differences in addition to the statistically significant differences. CONCLUSION: Reliable measurements of pulmonary artery pressure and pulmonary capillary wedge pressure cannot be obtained with patients in the 60 degrees lateral position.