Testicular adrenal rest tumors (TART) occur frequently in adolescents and adults with 21-hydroxylase deficiency. There have been no reports of TART in children with 3β-hydroxysteroid dehydrogenase deficiency (HSD3β). Biopsy proven TART was diagnosed in a 31/12-year-old male patient and also in his 22-month-old sibling. Hormonal and anthropometric measurements were performed during glucocorticoid and fludrocortisone treatment. The mutational analysis was performed by direct DNA sequencing of the complete coding region of the HSD3β2 gene. Initially, both siblings were treated with high doses of hydrocortisone and fludrocortisone. TART regressed with dexamethasone treatment in both patients. However, growth velocity decreased and weight gain increased in both patients. Dexamethasone was changed to high-dose hydrocortisone (>20 mg/m2/d). Sequencing analyses revealed a novel homozygous p.W355R (c.763 T>C) mutation at exon 4 of the HSD3β2 gene in both siblings. These two patients are, to our knowledge, the first known cases of TARTs with a novel mutation in the HSD3β2 gene detected during childhood. High-dose hydrocortisone treatment is more reliable for TART in children.