e23012 Background: Pulmonary metastases still remains the leading cause of mortality in osteosarcoma, with a 5-year survival rate of less than 30%. 17-β estradiol and testosterone are known to promote tumorigenesis and metastasis in other cancers, but its role in osteosarcoma tumorigenesis and metastasis is unclear. An understanding of the mechanism that these hormones have in osteosarcoma tumorigenesis and metastasis will lead to new therapeutic strategies using currently available targeted therapies used in breast and prostate cancer. Methods: We began by assessing the in vitro characteristics of two human osteosarcoma cell lines treated with 17-β estradiol or testosterone via proliferation and invasion assays. For the in vivo experiments, NCR nu/nu mice were injected with two luciferase-tagged human osteosarcoma cell lines and treated with 17-β estradiol and testosterone slow-release pellets. We first injected the mice subcutaneously and monitored tumor growth via caliper measurements. In the second experiment we injected the cell lines intratibially and monitored primary tumor growth in the bone and monitored metastasis formation in the lungs via IVIS bioluminescent imaging. Quantification of luciferase signal (photons/second) in the lungs was performed via the Xenogen Living Image software. H&E and immunohistochemistry for luciferase expression was performed on lung tissue to quantify tumor burden in the lungs. Statistical analysis was performed using Graphpad Prism. Results: Treatment with 17-β estradiol and testosterone increased cell proliferation and invasion in a dose-dependent manner. We observed increased tumor growth with both 17-β estradiol and testosterone in the subcutaneous model. In the intratibial experiment, testosterone increases primary tumor growth, but has no effect on metastasis. 17-β estradiol decreases primary tumor growth, but increases metastasis in a cell line independent manner. Conclusions: This study demonstrates a novel role for the puberty-related sex steroid hormones, 17-β estradiol and testosterone, in osteosarcoma tumor growth and metastasis. Uncovering the mechanism behind this phenomenon will uncover new therapeutic options for this devastating disease.