Just found a very interesting article from a couple of folks in California that studied the number of Moyamoya cases in the US from 1994-2004.

Their study stated they studied 2,247 cases in the US during that time period.  I looked up the population of the US during that time frame and it averaged approx 269 million.  That puts the prevalence of Moyamoya in the US at approx 1 in 120,000 for the years between 1994-2004.  MUCH smaller than originally estimated by the University of Iowa study in 1994.

Have a read:
http://www.frontiersin.org/endovascular_and_interventional_neurology/10.3389/fneur.2011.00043/full

DJ