Congenital adrenal hyperplasia (CAH) is an autosomal recessive disorder, and occurs in approximately 1 in 15000 births globally.   There are multiple forms of CAH, broken down into classical and nonclassical forms based on the amount of function retained. The classical forms include salt-wasting (SW), and simple-viralizing (SV). Mutations that interfere with the active site—the heme group or residues involved in substrate binding—result in a complete loss of enzymatic activity, the salt-wasting type.  Cortisol and aldosterone deficits are associated with life-threatening salt-loss (hence salt-wasting), as the steroids play roles in regulating sodium homeostasis . Retaining minimal enzyme activity, the simple-viralizing type is associated with mutations in conserved hydrophobic regions or near the transmembrane domain. Simple viralizing CAH patients maintain adequate sodium homeostasis, but exhibit other phenotypical symptoms shared by SW, including accelerated growth in childhood and ambiguous genitalia in female neonates. Nonclassical forms retain 20-60% of hydroxylase function—this form is associated with normal cortisol expression, but an excess of androgens post-puberty.  
Stikkelbroeck et al. (2001) investigated the prevalence of testicular tumors in 17 adolescent and adult male patients with CAH aged 16 to 40 years. In 16 of 17 patients, one or more testicular tumors ranging in maximal length from to cm were found on ultrasonography. In 6 patients, the testicular tumors were palpable. Undertreatment, defined as the presence of a salivary androstenedione level above the upper reference morning level, was found in 5 of 17 patients at the time of investigation. The other 12 patients were treated adequately or even overtreated at the time of investigation. Nevertheless, 11 of these 12 patients showed testicular tumors on ultrasonography. Tumor size was significantly larger in patients who were heterozygous or homozygous for deletion or conversion of the CYP21 gene than in patients who did not have this genotype. Impairment of Leydig cell function as manifested by decreased plasma levels of testosterone was found in 6 of 17 patients. Semen analysis in 11 patients revealed azoospermia in 3 patients and poor semen quality in 4 patients. The authors concluded that, when carefully sought for, testicular adrenal rest tumors are frequently present in adolescent and adult males with CAH and are often accompanied by impaired spermatogenesis and Leydig cell failure.