Abstract Objectives Male patients with congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency (21OHD) are at risk of developing testicular adrenal rest tumors (TART), which are benign but an important cause of infertility. The limited medical infrastructure and lack of knowledge and awareness of 21OHD in Indonesia has contributed to missed or delayed diagnoses, leading to a reportedly low prevalence of 21OHD and an expected high rate of undiagnosed patients. In Indonesia, TART has not been widely reported, with only a single patient documented in a recent paper. This study aim was to define the potential risk factors for developing TART in Indonesian pediatric patients with 21OHD. Methods This cross-sectional study was conducted in 20 boys with genetically proven 21OHD (0–18 years old). All had pathogenic variants associated with 0 % or 0–1% residual enzymatic activity. Medical history, clinical characteristics, genetic analysis results, biochemical measurements, bone age, and scrotal ultrasonography (US) data were evaluated. Results In 5 of 20 patients (25 %), TART was detected by US. The patients with TART were mainly in puberty (p=0.014). Higher androstenedione levels and advanced bone age were associated with TART. Conclusions TART is an important complication in Indonesian patients with 21OHD presenting before or during puberty. Factors associated with TART are pubertal age and longstanding poor hormonal control. Early diagnosis and optimization of treatment may help to prevent TART development and consequently improve fertility in these patients.