In an epidemiological study, infants identified at birth as having sex chromosome anomalies are enrolled in a voluntary, long term evaluation program. Case histories are presented of the first four boys in the series to have a 47, XXY karyotype. They have been followed from birth for 6 to 9 years, with physical and psychological evaluations. Parents were informed in general terms of the child's genetic defect, and were offered continuing support and encouragement throughout the study. Close cooperation with the families was maintained. So far, the development of all these children has fallen clearly within the normal range and a reasonable healthy developmental pattern has been secured. Minor deviations in motor, speech and emotional development suggest a common underlying pattern, but four cases are too few on which to establish a relationship between karyotype and phenotype. The data suggest that the symptomatology reported in selected children with a 47, XXY karyotype may be strongly dependent on factors other than the chromosomal constitution, and that an appropriate familial and environmental situation may minimize elevated risks due to the marked genetic defect.