Transcranial Doppler ultrasound has revealed the existence of cerebrovascular microemboli in asymptomatic patients with prosthetic heart valves. We investigated the relation between the presence and number of emboli signals and valve type. Patients with six types of prosthetic valves (Björk-Shiley monostrut, Medtronic-Hall, Carbomedics, ATS, Carpentier-Edwards standard, Carpentier-Edwards supraannular) were examined using transcranial Doppler ultrasound in two centers. The monitoring time was 30 min over the right middle cerebral artery. All patients were stabilized on warfarin at the time of study. Microemboli signals were identified by their characteristic audiovisual signal and on subsequent spectral analysis, based on accepted criteria. A standard neurologic questionnaire was completed by all patients. The prevalence of microemboli signals varied between 49% (Medtronic Hall) and 97% (Björk-Shiley monostrut), while their number varied between 1 [0-3] (Carpentier-Edwards standard) and 187 [136-240] (Björk-Shiley monostrut) per hour (median and 95% CI). Both parameters were significantly higher in patients with Björk-Shiley monostrut valves compared to the other patient groups. There were no significant differences in the prevalence of neurologic complications among the groups examined (overall 16%), or in emboli numbers between symptomatic and asymptomatic patients. The prevalence and quantity of microemboli signals in patients with prosthetic heart valves, as detected by transcranial Doppler, is dependent upon valve type. The clinical significance of these microemboli signals remains to be further evaluated.