Intelligence is reported to decline after onset of moyamoya in Japanese populations, but there is less evidence for this in Western populations where the condition may be secondary to stroke and sickle cell anaemia (SCA). Preoperative longitudinal IQ data were obtained from 15 children (seven males, eight females) who developed moyamoya syndrome (MMS) following a stroke (six with SCA, nine without SCA), and 19 controls (10 males, nine females; nine healthy control participants, 10 with SCA). At baseline assessment (Time 1) median age of patients was 7 years 6 months (range 3y 7mo to 12y 5mo); median age of controls was 6 years 3 months (range 4y to 11y 6mo). At follow-up (Time 2), ages were 11 years 8 months (range 3y 7mo to 12y 5mo) and 12 years 8 months (range 6y 4mo to 16y 8mo) in patients and controls respectively. Median duration of follow-up for the patient group was 3 years (range 7 to 10y) and in controls, 4 years 1 month (range 1 to 10y). In children with SCA, Verbal and Performance IQs (VIQ and PIQ) were significantly lower than in controls at Time 1; there was an additional independent statistically significant reduction in PIQ associated with MMS (p=0.004). Although there were further significant reductions in IQ by the second assessment for patients with MMS compared with controls, IQ did not differ significantly between groups with and without SCA. While the reduction in IQ attributed to SCA does not appear to become more marked with increasing age, the difference between those with and without MMS is associated with increasing effect over time.