Insulin-like factor 3 (INSL3) is produced primarily by testicular Leydig cells and acts by binding to its specific G-protein-coupled receptor, RXFP2 (relaxin family peptide 2). INSL3 is involved in testicular descent during fetal development, and mutations in the INSL3 and RXFP2 genes cause cryptorchidism. The physiological role of INSL3 in adults is not known, although substantial INSL3 circulating levels are present. After extensive clinical, biochemical, and hormonal investigations, including bone densitometry with dual energy X-ray absorptiometry, on 25 young men (age, 27-41 years) who have the well-characterized T222P mutation in the RXFP2 gene, we found that 16 of them (64%) had significantly reduced bone density. No other cause of osteoporosis was evident in these subjects, whose testosterone and gonadal function were normal. Expression analysis of INSL3 and RXFP2 on human bone biopsy and human and mouse osteoblast cell cultures performed by reverse transcription-PCR and immunohistochemistry showed the presence of RXFP2 in these cells. Real-time cAMP imaging analysis and proliferation assays under the stimulus of INSL3 showed a dose- and time-dependent increase in cAMP and cell proliferation, and specific osteoblast gene activation was observed by real-time PCR after INSL3 stimulation. Lumbar spine and femoral bone of Rxfp2-deficient mice were studied by static and dynamic histomorphometry and micro-computed tomography, respectively, and showed decreased bone mass, mineralizing surface, bone formation, and osteoclast surface compared to wild-type littermates, compatible with a functional osteoblast impairment. This study identified for the first time a role for INSL3 in adults, demonstrating a modulating effect on bone metabolism and linking RXFP2 gene mutations with human osteoporosis.