We analyzed the association between hematologic factors and blood velocity of the middle cerebral artery in 42 healthy ambulatory subjects aged 63-86 years. We found a significant inverse association between mean velocity and both hematocrit (r = -0.37, p less than 0.02) and fibrinogen concentration (r = -0.42, p less than 0.005). These two variables are independently associated with velocity and together explain 29% of the variance in mean velocity. Both hematocrit and fibrinogen concentration should be considered in the interpretation of transcranial Doppler findings in this population.