This double-blind, controlled study of XYY and XXY men found in a birth cohort of 4,591 tall men born in Copenhagen assessed evidence of delinquent and aggressive behavior and explored the role of hormonal determinants in the behavioral and psychological differences noted among groups. Information from social records, a structured psychological interview, and projective tests did not support the notion that men with sex chromosome anomalies are particularly violent or aggressive. Hormonally, XYY men had significantly higher concentrations of testosterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) than matched control groups. In contrast, XXY men had higher levels of LH, FSH, and prolactin, but low concentrations of testosterone. There was a significantly positive relation among all subjects as well as XY controls alone between plasma testosterone level and evidence of criminal convictions. A proportionate increase in testosterone levels was noted when subjects were divided into nondelinquents, delinquents without violent convictions, and delinquents with violent convictions. The relation between testosterone level and criminal behavior was not reflected in measures of aggression derived from the psychological interview and projective tests. There was no specific evidence that testosterone is a mediating factor in the criminal behavior of XYY men.