OBJECTIVES: Several cases of testicular adrenal rest tumours have been reported in men with congenital adrenal hyperplasia (CAH) due to the classical form of 21-hydroxylase deficiency but the prevalence has not been established. The aims of this report were to evaluate the frequency of testicular adrenal rest tissue in this population in a retrospective multicentre study involving eight endocrinology centres, and to determine whether treatment or genetic background had an impact on the occurrence of adrenal rest tissue. MATERIAL AND METHODS: Testicular adrenal rest tissue (TART) was sought clinically and with ultrasound examination in forty-five males with CAH due to the classical form of 21-hydroxylase deficiency. When the diagnosis of testicular adrenal rest tumours was sought, good observance of treatment was judged on biological concentrations of 17-hydroxyprogesterone (17OHP), delta4-androstenedione, active renin and testosterone. The results of affected and non-affected subjects were compared. RESULTS: TART was detected in none of the 18 subjects aged 1 to 15years but was detected in 14 of the 27 subjects aged more than 15years. Five patients with an abnormal echography result had no clinical signs. Therapeutic control evaluated at diagnosis of TART seemed less effective when diagnosis was made in patients with adrenal rest tissue compared to TART-free subjects. Various genotypes were observed in patients with or without TART. CONCLUSION: Due to the high prevalence of TART in classical CAH and the delayed clinical diagnosis, testicular ultrasonography must be performed before puberty and thereafter regularly during adulthood even if the clinical examination is normal.