Klinefelter's syndrome (XXY syndrome) has been defined as the spectrum of phenotypic features resulting from a sex chromosome complement that includes two or more X chromosomes and one or more Y chromosomes. Cytogenetic surveys conducted across the world have identified a sizable population of XXY males, who have been studied extensively from the newborn period through adolescence. The longitudinal studies of these boys have produced an accurate and reliable account of the growth and development of the XXY male. There now exists a growing body of knowledge that suggests that XXY boys often experience language deficits, neuromaturational lag, academic difficulties, and psychological distress, which may be reduced or ameliorated by early identification, anticipatory guidance, and proper medical management.