CONTEXT Infertility observed in adult males with congenital adrenal hyperplasia (CAH) has been associated with testicular adrenal rest tumors (TART) that may originate during childhood. OBJECTIVE Our objective was to describe the prevalence of TART and Sertoli and Leydig cell function in a group of boys aged 2-10 yr with CAH and to compare prevalence with that of a control group. DESIGN From August 2005 to January 2007, 19 patients with classical CAH (CAH group) were referred from seven endocrinology centers. METHODS We studied 19 subjects in the CAH group and, as a control group, 13 boys from the community that did not have testicular diseases. A complete physical exam was performed. High-resolution ultrasound was used to determine TART prevalence. Inhibin B and anti-Müllerian hormone were used as Sertoli cell markers. The ratio between basal testosterone levels and testosterone levels 72 h after beta-human chorionic gonadotropin (5000 U/m2) treatment [(T72- T0)/T0] was used to evaluate Leydig cell response. RESULTS CAH and control groups were comparable in chronological age (5.9 vs. 5.6 yr; P = 0.67) and bone age/chronological age ratio (1.09 vs. 1.03; P = 0.09). TART prevalence was four of 19 (21%) in the CAH group. Lower values for inhibin B (49.2. vs. 65.2 pg/ml; P = 0.018), anti-Müllerian hormone (70.1 vs. 94.2 ng/ml; P = 0.002), and (T72- T0)/T0 (5.6 vs. 13.6; P < 0.01) were observed in the CAH group. CONCLUSION TART in prepubertal males with classic CAH could be found during childhood. We also report differences in markers of gonadal function in a subgroup of patients, especially in those with inadequate control.