Analysis of 819 participants enrolled in the Rett syndrome (RTT) Natural History Study validates recently revised diagnostic criteria. 765 females fulfilled 2002 consensus criteria for classic (653/85.4%) or variant (112/14.6%) RTT. All participants classified as classic RTT fulfilled each revised main criterion; supportive criteria were not uniformly present. All variant RTT participants met at least 3 of 6 main criteria in the 2002, 2 of 4 main criteria in the current format, and 5 of 11 supportive criteria in both. This analysis underscores the critical role of main criteria for classic RTT; variant RTT requires both main and supportive criteria. Copyright © 2010 American Neurological Association.