PURPOSE: This study aims to describe the cohort of Adolescent Medicine Trials Network for HIV/AIDS Interventions (ATN) research program participants and evaluate whether the ATN's recently completed 5-year cycle recruited study participants who parallel the populations most impacted by HIV in the United States. METHODS: Harmonized measures across ATN studies collected at baseline were aggregated for participants aged 13-24 years. Pooled means and proportions stratified by HIV status (at risk for or living with HIV) were calculated using unweighted averages of study-specific aggregate data. Medians were estimated using a weighted median of medians method. Public use 2019 Centers for Disease Control and Prevention surveillance data for state-level new HIV diagnoses and HIV prevalence among US youth aged 13-24 years were obtained for use as reference populations for ATN at-risk youth and youth living with HIV (YLWH), respectively. RESULTS: Data from 3,185 youth at-risk for HIV and 542 YLWH were pooled from 21 ATN study phases conducted across the United States. Among ATN studies tailored to at-risk youth, a higher proportion of participants were White and a lower proportion were Black/African American and Hispanic/Latinx compared to youth newly diagnosed with HIV in the United States in 2019. Participants in ATN studies tailored to YLWH were demographically similar to YLWH in the United States. DISCUSSION: The development of data harmonization guidelines for ATN research activities facilitated this cross-network pooled analysis. These findings suggest the ATN's YLWH are representative, but that future studies of at-risk youth should prioritize recruitment strategies to enroll more participants from African American and Hispanic/Latinx populations.