In most clinical conditions pulmonary artery (PA) wedge pressure accurately reflects left ventricular (LV) end-diastolic pressure. In the presence of mitral regurgitation (MR), large V waves can distort PA wedge pressure and result in incorrect estimation of LV end-diastolic pressure. In 52 patients with MR simultaneous measurement of PA wedge pressure or left atrial pressure and LV end-diastolic pressure was recorded. Twenty-one (40%) patients had large V waves (V wave greater than A wave by greater than 10 mm Hg, group 1), and 31 (60%) patients had small V waves (group 2). Group 1 had significantly higher V waves than group 2 (46 +/- 3 vs 21 +/- 2 mm Hg, p less than 0.001). The LV end-diastolic pressure was similar in both groups (21 +/- 2 vs 19 +/- 2 mm Hg, difference not significant). The mean PA wedge or left atrial pressure in group 1 (26 +/- 2 mm Hg) overestimated LV end-diastolic pressure (21 +/- 2 mm Hg) by 30% (p less than 0.01), but the trough of the X descent (20 +/- 2 mm Hg) was similar to the LV end-diastolic pressure. In group 2 patients with small V waves the mean PA wedge pressure was not significantly different from the LV end-diastolic pressure (16 +/- 2 vs 19 +/- 2 mm Hg, p = 0.06), but the trough of the X descent (13 +/- 2 mm Hg) underestimated LV end-diastolic pressure.(ABSTRACT TRUNCATED AT 250 WORDS)