Insulin-like factor 3 (INSL3) is a peptide hormone produced in leydig cells of the testes. Its role in the adult male is unknown but INSL3 and its receptor RXFP2 have been linked to bone cell differentiation. It is speculated that low levels of INSL3 could be responsible for low bone mineral density in patients with primary osteoporosis and Klinefelter's Syndrome. The aim of this study was to assess plasma INSL3 in patients with osteoporosis and Klinefelter's Syndrome compared to healthy males. Fourteen healthy males, 21 males with osteoporosis (4 primary and 17 secondary) and 4 patients with Klinefelter's Syndrome were studied. Plasma INSL3, testosterone, LH, FSH and Sex hormone-binding globulin were evaluated. Plasma INSL3 concentrations were similar in osteoporosis patients compared to healthy controls (0.72 vs. 0.69 ng/mL, p = 0.26). INSL3 was significantly higher in patients with primary osteoporosis (n = 4) compared to age-matched healthy controls (n = 8) (0.845 vs. 0.665 ng/mL, p = 0.021). INSL3 levels in Klinefelter's Syndrome patients were significantly lower compared to healthy controls (0.39 vs. 0.69 ng/mL, p = 0.01). Plasma INSL3 levels were lower in Klinefelter's Syndrome reflecting testicular failure. INSL3 levels were not lower in men with osteoporosis. The relationship between INSL3, its receptor and bone metabolism requires further study.