Children and adolescents with Klinefelter syndrome (XXY) have been reported to show deficits in language processing including VIQ < PIQ and a learning disability in reading and spelling. However, whether this is characteristic of adults with Klinefelter syndrome has not been established. Thirty-five men with Klinefelter syndrome, aged 16 to 61, and 22 controls were evaluated with a comprehensive neuropsychological battery. The Klinefelter patients scored significantly below controls in language skills, verbal processing speed, verbal and nonverbal executive abilities, and motor dexterity. Within the Klinefelter sample, three cognitive subgroups were identified: VIQ 7 or more points below PIQ (n = 10), VIQ within 6 points of PIQ (n = 12), and PIQ 7 or more points below VIQ (n = 12). The deficits detected in language, verbal processing speed, and verbal executive skills were found to be isolated to the VIQ < PIQ subgroup, while the abnormalities in motor dexterity and nonverbal executive skills were confined to the PIQ < VIQ subgroup. Older age was significantly correlated with increases in VIQ relative to PIQ in the patient group, which suggests the intriguing possibility that the PIQ < VIQ subgroup primarily emerges in young adulthood, perhaps in response to the reported hormonal abnormalities detected in Klinefelter syndrome patients during puberty. (JINS, 2001, 7, 446–456)