This report presents data defining the neuropsychological and cognitive phenotypes of a group of adults with sex chromosome abnormalities identified at birth through the chromosome screening of 40,000 consecutive newborns between 1964 and 1974. In all three nonmosaic groups, reading skills were impaired and intelligence quotients were on average reduced more than 20 points relative to controls. The 47,XXX women demonstrated greatest overall impairment, including reduced scores on tests of conceptualization and problem solving. 45,X women demonstrated impairment in spatial thinking skills, and 47,XXY men in verbal processing skills. No reduced scores were found in the female mosaic group. Marked variability in scores was seen in all groups; some propositi have been unable to hold any job, whereas others have completed college and are professionally employed. © 2001 Wiley-Liss, Inc.