Using multiplane transoesophageal echocardiography (TOE), we investigated the haemodynamic response to acute normovolaemic haemodilution (ANH) in anaesthetised patients with critical aortic stenosis. Twenty-eight patients were randomly assigned to ANH or control groups. In the control group, haemodynamic data remained unchanged over a 20-min period. In the ANH group, haemoglobin levels decreased from a mean (SD) of 134 (7) to 91 (9) g x l(-1) (p < 0.001) whereas stroke volume, central venous pressure and left ventricular (LV) end-diastolic area all increased significantly (mean (SD) +15 (6) ml; +2.0 (1.1) mmHg; +2.1 (0.8) cm2, respectively). During ANH, the accelerated blood flow through the stenotic valve caused an increased loss (SD) in LV stroke work: from 24 (8)% to 30 (10)%), (p < 0.01). Hence, lowering viscosity with ANH resulted in improved venous return, higher cardiac preload and increased stroke volume. However, this adaptive haemodynamic response was limited by less efficient LV stroke work due to dissipation of fluid kinetic energy.