High intensity transcranial Doppler signals (HITS), assumed to be caused by microemboli, have been reported to occur in many patients with mechanical heart valve prostheses. The aim of our study was to quantify these phenomena and to find possible differences. Furthermore, parameters which might influence the prevalence of HITS were investigated. Monitoring of both middle cerebral artery frequency shift spectra was carried out for 10 minutes in 100 patients having an aortic (n = 64) and (n = 5)/or (n = 31) mitral mechanical heart valve prosthesis. The spectra were off-line screened for HITS by ear. The findings were correlated with the degree of anticoagulation and with the time period since implantation. To examine if platelet aggregates would be the underlying cause, another HITS count was done for 10 minutes prior to and 40 minutes after i.v. injection of 250 mg aspirin (ASA) as well as after four days of 100 mg/day ASA orally in a group of seven patients. Prior to surgery, HITS were present in only one patient. Postoperatively, HITS were detected in 54 of 100 patients. There was no significant difference between left and right sides, no correlation with anticoagulation, and only a borderline correlation with the time interval since implantation. Sixty-six percent of the 50 patients monitored within the first three postoperative weeks had HITS as compared to only 42% in 50 subjects who were examined three months or later after surgery (p < 0.05). With regard to the number of HITS an inverse behaviour was observed.(ABSTRACT TRUNCATED AT 250 WORDS)