Congenital adrenal hyperplasia (CAH) is a recessive condition that affects the adrenal glands. Despite life-long replacement therapy with glucocorticoids and mineralocorticoids, adult patients with CAH often experience impaired gonadal function. In pubertal boys and in men with CAH, circulating testosterone is produced by the adrenal glands as well as the testicular, steroidogenic cells. In this European two-center study, we evaluated the function of Leydig and Sertoli cells in 61 boys and men with CAH, primarily due to 21-hydroxylase deficiency. Despite conventional hormone replacement therapy, our results indicated a significant reduction in serum concentrations of both Leydig cell-derived hormones (i.e. insulin-like factor 3 (INSL3) and testosterone) and Sertoli cell-derived hormones (i.e. inhibin B and anti-Müllerian hormone) in adult males with CAH. Serum concentrations of INSL3 were particularly reduced in those with testicular adrenal rest tumors. To our knowledge, this is the first study to evaluate circulating INSL3 as a candidate biomarker to monitor Leydig cell function in patients with CAH.