Insulin-like 3 (INSL3) is a hormone produced by Leydig cells (LCs) and leads to physiological testicular descent during embryonic development. We investigated the expression of INSL3 by immunohistochemistry in normal LCs, in Leydig cell tumor (LCT) (n=17 including 15 testes and 2 ovaries) and in Leydig cell hyperplasia (LCH) (n=10). Normally distributed LCs showed strong immunostaining in the cytoplasm in all cases. All 10 cases (100%) of LCH were strongly and diffusely positive in the intertubular areas. Six cases of LCH had nodules raging in size from 0.2 to 0.9 cm with variable INSL3 staining. Fifteen of 17 (88.2%) LCTs showed marked decrease INSL3 staining, 10/17 (58.8%) were completely negative, and 5/17 (29.4%) were only focally positive. Two cases with multifocal LCTs showed strong and diffuse cytoplasmic staining of LCs around seminiferous tubules while the LCTs were negative. Two cases diagnosed as LCT were strongly positive for INSL3. Other sex cord stromal tumors tested were consistently negative including Sertoli-cell tumor (n=4), granulosa cell tumor (n=2), and fibrothecoma (n=1). In conclusion, our results contrast with those of previously published studies, and show that the great majority of LCTs are negative or have decreased expression of INSL3 while its expression is retained in LCH. INSL3 negative nodules within LCH may represent early LCTs. INSL3 immunostaining could be helpful to highlight LCs in cases where it is difficult to identify them (ie, small testicular biopsies performed for infertility workup) and in the differential diagnosis between florid LCH and LCT.