Strokes occur in children with SCA primarily due to large vessel occlusive disease.! Moyamoya is an angiographie pattern consisting of large vessel occlusion and ai telangiectatic network of collateral vessels often seen in these patients, but its significancei is unkonwn. After an initial stroke 65% patients experience recurrences. Chronic] transfusions decrease the risk of ischémie stroke but not hemorrhages or transient ischémie! attacks (TIAs). Risk factors for recurrent CVA are poorly understood. We studied 55! patients (18F.37M) with HbSS who had infarctive strokes and were transfused to maintain! HbS below 30%. They were followed for a mean of 8.0 years (0.7 to 20.4 years). One patient who moved was excluded from analysis. MR1 or CT was performed in all patientsj after the first episode. All patients subsequently underwent either conventional or magnetic! resonance angiography (MRA). The MR1 and MRA studies were repeated after each neurologic event. Twenty two of 54 (41%) patients (6M,16F)were found have moyamoyal collaterals. The mean age at time of the first CVA was 7.4 years (rangel.7 to 16.9) for] patients without compared to 8.3 years (range 1.1 to 18) for patients with moyamoya. Baseline hématologie values were not different between the two groups. Of the patients! with moyamoya 11 (50%) experienced 20 recurrencesfinfarcts or TlAs)while 6 of 32i (18.8%) patients without moyamoya experienced 7 recurrent events (p<0.02). Moyamoya| patients were more likely to have subsequent infarctive strokes (13.6% vs. 2.9%; p<0.05)i or TIAs (40.9% vs. 8.8%; p<0.05) compared to patients without moyamoya, no, hemorrhages were documented. The mean recurrence-free survival was significantly shorteri for patients with moyamoya than for those without (31.1 months vs. 125.8 months; p<0.02. The mean time to recurrence was 1.2 years (range 0.04 to 4.3 years),in the moyamoya group compared to 3.7 years (range 0.3-13.3 years) for patients without moyamoya. In our series of patients a significantly higher number of recurrent neurologic j events occurred in patients with moyamoya collaterals. These findings suggest that these patients are responsible for most recurrences and should be studied separately when new treatment modalities such as hydroxyurea are used in this patient population.