Cross-sex hormone therapy (XHT) is widely used by transgender people to alter secondary sex characteristics to match their desired gender presentation. Here we investigate the long-term effects of XHT on bone health, using a murine model. Female mice underwent ovariectomy at either 6 or 10 weeks, and began weekly testosterone or vehicle injections. Dual-energy X-ray Absorptiometry (DXA) was performed to measure bone mineral density (BMD) and microCT was performed to compare femoral cortical and trabecular bone architecture. The 6-week testosterone group had comparable BMD to controls by DXA, but reduced bone volume fraction, trabecular number, and cortical area fraction, and increased trabecular separation by microCT. 10-week ovariectomy/XHT maintained microarchitecture, suggesting that estrogen is critical for bone acquisition during adolescence, and that late, but not early, estrogen loss can be sufficiently replaced by testosterone alone. Given these findings, we then compared effects of testosterone to effects of weekly estrogen or combined testosterone/low-dose estrogen treatment after a 6-week ovariectomy. Estrogen treatment increased spine BMD and microarchitecture, bone volume fraction, trabecular number, trabecular thickness, and connectivity density, and decreased trabecular separation. Combined testosterone-estrogen therapy caused similar increases in femur and spine BMD and improved architecture (increased bone volume fraction, trabecular number, thickness, and connectivity density) to estrogen therapy, and were superior compared to mice treated with testosterone-only. These results demonstrate estradiol is critical for bone acquisition and suggest a new cross-sex hormone therapy adding estrogens to testosterone treatments, with potential future clinical implications for treating transgender youth or males with estrogen deficiency