Developmental outcome in 49,XXXXY Klinefelter syndrome.

The developmental histories of two males who have 49 XXXXY Klinefelter syndrome are described. Now aged 16 and six, they have been followed since the ages of four and two, respectively. They have many of the typical physical characteristics described in the literature, but their mental retardation is not as severe as has been reported. Both are moderately delayed in their general development and their personalities and learning styles are more similar to XXY Klinefelter individuals. These two case studies demonstrate previously unreported potential in individuals with this disorder, and the authors discuss the implications of this finding.