Behavioral investigation of men with sex chromosome anomalies has been primarily limited to the study of institutionalized individuals or patient groups. A double-blind controlled investigation of XYY and XXY men found in a birth cohort of 4591 tall men born in Copenhagen gathered sexual information and assessed the role of hormonal determinants on sexual behavior. There were significant differences in several sexual dimensions and in gender role between XYY men and their controls and XXY men and their controls as well as between XYY and XXY men. Although both proband groups differed from each other and from their controls in pituitary gonadal function, there was no evidence that adult hormonal levels mediate the effect of sex chromosome anomalies on male sexuality.