 This research study in infant males with Klinefelter syndrome 47,XXY will learn more about the effect of testosterone on early health and development. The study is a total of three visits over 6 months with assessments of motor skills, body composition muscle and fat, and hormone levels. This is a randomized, placebo-controlled study but all infants will receive testosterone treatment during the study period. The investigators will learn how testosterone treatment in infancy effects short term outcome measures on health and development. 