 In typical male development, there is a surge of testosterone in the first few months of life, commonly known as the mini-puberty period of infancy. This testosterone surge may be critical for neurodevelopmental and cardiometabolic programming throughout life. Recently there has been increased off-label use of testosterone in infants with XXY, however neither the short or long term safety or efficacy have been evaluated. This study aims to quantify the short term effects of testosterone treatment in infants with XXY on neurodevelopment, growth, body composition, testicular function, and safety parameters. This is a double blind randomized placebo controlled trial of testosterone injections 25 mg every 4 weeks for 3 doses in boys with XXY enrolled between 1 and 3 months of age. Outcomes including body fat percentage, scaled motor developmental scores, growth velocity, testicular hormone concentrations, specific metabolites, and safety parameters will be assessed 12 weeks into the study. The groups will then cross-over all subjects will receive testosterone during the study period and the outcomes will be reassessed 24 weeks into the study. The secondary questions the investigators will answer with this cross-over is 1 whether benefits in the treatment group at 12 weeks are sustained at 24 weeks, and 2 whether the same benefits are seen if treated after the mini-puberty period. 