Haemodynamic parameters and oxygen consumption were determined in 20 patients with mitral regurgitation before and after a 12 ml.kg-1 isovolaemic exchange of blood for 6% hydroxyethyl starch. During haemodilution, mean (SEM) haemoglobin concentration decreased from 13.0 (0.4) to 10.3 (0.4) g.dl-1 (p = 0.001). With cardiac filling pressures maintained at predilution levels, cardiac index increased from 1.84 (0.08) to 1.94 (0.08) l.min-1.m-2 (p = 0.025) while systemic vascular resistance decreased from 1556 (86) to 1425 (83) dyne.s.cm-5 (p = 0.002) and oxygen extraction increased from 31.7 (1.1) to 37.3 (1.4)% (p = 0.001) resulting in an unchanged oxygen consumption. The haemodynamic response to haemodilution was not affected by the patients' cardiac rhythm, i.e. whether it was sinus rhythm or atrial fibrillation. In conclusion, isovolaemic haemodilution to a haemoglobin of 10.3 g.dl-1 is well tolerated in patients with mitral regurgitation. Compensatory mechanisms include both an increase in cardiac index and an increase in oxygen extraction.