 XXY also known as Klinefelter syndrome is the most common chromosomal abnormality in males, affecting 1600 boys. The extra X chromosome leads to insufficient development of the testicles and subsequent testosterone deficiency. Males with XXY also have a high risk for developmental delays, learning disabilities, and cardiovascular disease. An essential question is how much of this risk is because of testosterone deficiency and could therefore be reduced by testosterone supplementation, particularly during critical periods of development. 