The hydrodynamic and haemodynamic function of 29 mm mitral and 23 mm aortic St Jude Medical (SJM) and CarboMedics (CM) heart valves have been compared in a pulsatile flow simulator in the laboratory and by doppler ultrasound in vivo. The laboratory studies showed that there was no significant difference in the pressure drop across the valves. The valves also had similar regurgitant volumes. Doppler ultrasound confirmed these results with a mean pressure drop of 12.9 +/- 5.5 mmHg across the CM aortic valves and 12.6 +/- 5.5 mmHg for the SJM aortic valves. The mean mitral diastolic pressure drops were 3.8 +/- 1.8 and 4.6 +/- 1.4 mmHg for the CM and SJM valves, respectively. Mild aortic regurgitation was detected in 5 of 14 patients in the CM group and 1 out of 6 patients in the SJM group, though none of the cases were clinically significant. Both the in vitro and in vivo results show these valves to have acceptable and comparable function.