Twelve XXY boys identified at birth in a population screening study were followed up at a mean age of 16.7 yr and compared with a control group of chromosomally normal boys matched for age and social class. Assessment of personality and psycho-sexual development showed the XXY boys to be more timid and less self-confident and to have problems in relating to their peer group than the controls. They also showed some evidence of delayed or impaired development of sexual interest. In general, the differences were not marked. Possible causes for these differences and potential methods of preventing them are discussed.