Background: Klinefelter’s syndrome (KS) is characterized by extra X chromosomes and features of primary hypogonadism including osteopenia and osteoporosis. Testosterone therapy (TTh) is widely used to treat men with KS and low serum testosterone/hypogonadal symptoms, though studies on its efficacy in improving bone density show varied outcomes. Materials and Methods: We studied the effects of TTh, bisphosphonates, and vitamin D/calcium in 38 men with KS and low testosterone, hypogonadal symptoms, and T-scores consistent with osteoporosis. Our aim was to investigate at the end of follow-up (median: 87 months, range: 27–147 months), associations between age, baseline total testosterone, and T-scores, and change in T-scores after treatment. Results: At final assessment, all men had T-score values outside the osteoporotic range (1.1 standard deviation [SD], 1.8 SD). Baseline age but not median baseline testosterone appeared associated with change in T-score and T-score at final assessment. All men had dual-energy X-ray absorptiometry every 6 months and demonstrated continued improvement in T-scores after 3 months and up to 72 months. Baseline age and T-scores (stratified by median) were associated with change in T-score at final assessment. Compared with men ‡51 years, those aged <51 years showed significantly greater improvement in T-scores between 6 and 30 months. Men with worse T-score values (<3.7 SD) showed significantly greater improvement at every time point up to 36 months. Our results indicate that TTh, bisphosphonates, and vitamin D/calcium improve osteoporosis although there is a need to better understand the effects of the individual therapies, age, and baseline T-score on treatment efficacy.