STUDY QUESTION: What is the long-term outcome of individuals born with bilateral testicular regression (BTR) in relation to its underlying etiology? SUMMARY ANSWER: Statural growth and pubertal development are adequate with incremental doses of testosterone replacement therapy (TRT); however, penile growth is often suboptimal, especially in those with a suspected genetic etiology (i.e. heterozygous DHX37 variants) or a micropenis at birth. WHAT IS KNOWN ALREADY: BTR is a rare and poorly understood condition. Although a vascular origin has been postulated, heterozygous missense variants in DHX37 have been attributed to the phenotype as well. How these various etiologies impact the clinical phenotype, gonadal histology and outcome of BTR remains unclear. STUDY DESIGN, SIZE, DURATION: For this cross-sectional study, individuals with BTR were recruited in eight Belgian pediatric endocrinology departments, between December 2019 and December 2022. A physical exam was performed cross-sectionally in all 17 end-pubertal participants and a quality of care questionnaire was completed by 11 of them. Exome-based panel testing of 241 genes involved in gonadal development and spermatogenesis was performed along with a retrospective analysis of presentation and management. A centralized histological review of gonadal rests was done for 10 participants. PARTICIPANTS/MATERIALS, SETTING, METHODS: A total of 35 participants (33 with male, 1 with female, and 1 with non-binary gender identity) were recruited at a mean age of 15.0 ± 5.7 years. MAIN RESULTS AND THE ROLE OF CHANCE: The median age at presentation was 1.2 years [0–14 years]. Maternal gestational complications were common (38.2%), with a notably high incidence of monozygotic twin pregnancies (8.8%). Heterozygous (likely) pathogenic missense variants in DHX37 (p.Arg334Trp and p.Arg308Gln) were found in three participants. No other (likely) pathogenic variants were found. All three participants with a DHX37 variant had a microphallus at birth (leading to female sex ...