We have recorded blood flow velocity in the anterior and middle cerebral arteries by transcranial Doppler sonography in abstinent marijuana abusers (n = 16) and control subjects (n = 19) to assess the effects of prolonged marijuana use of the cerebrovascular system. The pulsatility index, a measure of cerebrovascular resistance, and systolic velocity were significantly (p < 0.005) increased in marijuana abusers compared to the control subjects. These findings suggest that cerebral perfusion observed in 18-30 year old marijuana abusers is comparable to that of normal 60 year-olds. Thus, chronic abuse of marijuana might be a risk factor for stroke.