Testicular adrenal rest tumours (TARTs) have been described in patients with congenital adrenal hyperplasia (CAH). The aim of the study was to determine the prevalence of TARTs in patients with CAH, the associated factors and their impact on gonadal function. It is a prospective study concerning six young adult men with CAH, four cases with 21-hydroxylase deficiency and two cases with 11-hydroxylase deficiency. All patients were under glucocorticoid therapy. The mean age was 25 years (range: 20-31). All patients underwent a physical examination with testicular palpation, scrotal ultrasonography, a blood sample for serum testosterone, FSH, LH, inhibin B, ∆4-androstenedione and 17-OH-progesterone measurements and a semen analysis. Ultrasound revealed TARTs in four patients; three were bilateral. The mean tumour size was 6.3 ml (range: 0.02-14.1). The tumours were palpable in two cases. 17-OH-progesterone was <10 ng/ml in all cases. Decreased testosterone level was found in one case. The semen analysis revealed azoospermia in one case and poor semen quality in four patients. TARTs were common and associated with impaired spermatogenesis.