Testicular adrenal rest tumours (TART) are one of the most important causes of infertility in adult male patients with congenital adrenal hyperplasia (CAH). These benign tumours are already detected in children, but screening of TART is not routinely performed.To define retrospectively the prevalence of TART in 41 paediatric male CAH patients aged 0-19 years regularly followed by high-frequency (Fcentral 12-MHz) ultrasound techniques.Above the age of 10 years, there was a clear increase in the prevalence of TART: 10-12 years, 28\% (2 of 7 patients), 13-14 years, 50\% (4/8), and 15-16 years, 75\% (3/4). Above the age of 16 years, TART were detected in 100\% of the patients (7/7). The tumours were not detectable by palpation.TART is already present in childhood with an increasing prevalence after onset of puberty. We recommend regular ultrasound from the onset of puberty in all boys with classic CAH. © 2014 S. Karger AG, Basel.