The 29-year-old subject was examined a number of times in the Geneva psychiatric clinic over a 5-year period at the request of the courts. Other information derives from medical observations at the Bel-Air Medical Clinic where the subject was hospitalized three times and from interviews with the subject's father, mother, sister, and with the patient himself. The objective of the present study is to determine the accuracy of characteristics attributed in literature to the XYY syndrome. These are large size, subnormal intelligence, poor human contacts, excessive aggressiveness, absence of a sense of guilt or remorse, and electroencephalographic abnormalities. The subject is impulsive, unstable, and has been unruly from an early age, acting disruptive in school and failing to focus his attention on scholastic efforts or professional training. Because of electroencephalographic abnormalities, the subject has been diagnosed as epileptic. Despite intensive efforts of this family, his intellectual and moral development is retarded and he has repeatedly committed thefts. Examination in 1964 revealed large size, an IQ of 105, a need to focus attention on himself, egocentrism and instability, anxiety in his relationships with others, and a tendency to abuse alcohol. While serving a 7-year prison sentence, the subject's judgment, emotional development, and self-control improved. In the author's view, this example of the XYY syndrome proves the negative effect of the chromosomes on intellectual, emotional, and moral development, and confirms the notion of the consititutional psychopathy which predisposes the individual to crime. A bibliography of five entries and several illustrations are furnished. --in French.