The stroke prevention study in sickle cell disease (STOP) demonstrated a 90% reduction in stroke risk with transfusion among patients with time-averaged mean cerebral blood velocity (TAMV) of 200 cm/s or more as measured by transcranial Doppler (TCD). In STOP, 232 brain magnetic resonance angiograms (MRAs) were performed on 100 patients, 47 in the transfusion arm and 53 in the standard care arm. Baseline MRA findings were interpreted as normal in 75 patients and as indicating mild stenosis in 4 patients and severe stenosis in 21 patients. Among 35 patients who underwent magnetic resonance angiography within 30 days of random assignment, the TAMV was significantly higher in 7 patients with severe stenosis compared with 28 patients with normal MRA findings or mild stenosis (276.7 ± 34 vs 215 ± 15.6 cm/s; P < .001). In the standard care arm, 4 of 13 patients with abnormal MRA findings had strokes compared with 5 of 40 patients with normal MRA findings (P= .03). In this arm, TAMV became normal (less than 170 cm/s) or conditional (170-199 cm/s) in 26 of 38 patients with normal or mildly abnormal baseline MRA but remained abnormal in 8 of 10 patients with severely abnormal baseline MRA. These results suggest that TCD often detects flow abnormalities indicative of stroke risk before MRA lesions become evident. Furthermore, patients with abnormal MRA findings and higher TCD velocities are at higher risk for stroke, and their cerebral TAMVs are unlikely to decrease without transfusion. © 2004 by The American Society of Hematology.