Cytogenetic surveys of neonates have found that approximately one boy in 500 is born with an extra sex chromosome. Some of these boys are now being diagnosed when prenatal karyotyping is done for the detection of Down syndrome and other major aneuploidies. This study estimates what proportion of those not detected prenatally will be diagnosed postnatally and what the indications for karyotyping are likely to be. We ascertained all 47,XXY and 47,XYY males detected prenatally and postnatally (during the 4 years 1990-1993) in the three cytogenetic laboratories in the North Thames (West) region. The age at diagnosis and indication for karyotyping were noted for cases diagnosed postnatally. Less than 10 per cent of the estimated number of affected fetuses were detected prenatally. This study suggests that most males born with these chromosome patterns will go through life without being karyotyped, that the commonest indication for a 47,XYY male to be karyotyped will be developmental delay and/or behaviour problems, and that the commonest indication for a Klinefelter male to be karyotyped will be hypogonadism and/or infertility. It would appear that most undiagnosed 47,XXY and 47,XYY males do not look or behave in a manner which prompts testing for a chromosome abnormality.