To investigate the effect of hematocrit and serum fibrinogen on transcranial Doppler ultrasound (TCD) measurements, we performed, TCD tests and measured hematocrit and serum fibrinogen concentrations in 112 healthy adult volunteers. The mean velocities of the middle cerebral (r = -.37, p < .0001), internal carotid at siphon (r = -.14, p < .05) and cervical level (r = -.14, p < .05), vertebral (r = -.21, p < .005) and the basilar artery (r = -.33, p < .001) were significantly and inversely correlated with hematocrit, while serum fibrinogen weakly affected the mean velocities of the internal carotid artery. The subjects with low hematocrit (< or = 40%) showed significantly higher mean velocities of the middle cerebral, vertebral and basilar arteries than those with high hematocrit (> 40%). Considering the influence of the subject's age and gender, hematocrit is the strongest factor influencing the velocities of the middle cerebral and the basilar artery. These results suggest that hematocrit is an important variable for TCD measurements of the cerebral blood velocities and should be taken into account in TCD application.