Forty-two adult males - 10 with Klinefelter's syndrome (karyotype XXY), 17 with normal sex chromosome karyotype (XY) but with physical signs of hypogonadism, and 15 presumed to be normal both chromosomally and hormonally - completed tests of Verbal and Nonverbal ability. It was predicted: a) that the XXY group would perform more poorly than the other two groups on tests of Verbal ability, and b) that the XY, hypogonadal group would perform more poorly than the others on tests of Non-verbal ability. The first hypothesis was supported. However, in spite of measurable verbal deficits, the XXY men (whose parents were more highly educated than those of the other two groups) did not appear to be underachievers. Rather, they were, in general, as well educated, as likely to be employed, and of socioeconomic status equivalent to that of the other men in the study. These findings are discussed briefly in terms of implications for early childhood education and genetic counselling. The second hypothesis was not supported: this is attributed to presumed heterogeneity of etiology of hypogonadism in the XY, hypogonadal group.