Background: Research trial participation may influence health outcomes regardless of the intervention assigned but is often not assessed. Setting: We investigated how participation in an HIV prevention trial (the HIV Prevention Trials Network (HPTN) 068 study) affected health outcomes four years after the study in adolescent girls and young women (AGYW) in South Africa beyond effects of the tested intervention. Methods: We developed an analytical cohort that included the HPTN 068 trial participants from the Agincourt Health and Demographic Surveillance System (AHDSS) and resembled HPTN 068 trial enrollees (aged 13 to 20 years and in grades 8-11 in 2011) using inverse probability of treatment weights. We estimated risk differences (RD) for the association between trial participation and education and early parity (age <20 years) in 2019, after accounting for differences at baseline between the trial participants and non-participants. Results: There were 3,442 young women enrolled in grades 8-11 in 2011; 1,669 were in the HPTN 068 trial. Trial participants were more likely to have completed secondary school by 2019 (adjusted RD (aRD) 5.0%, 95% Confidence Interval (CI) 2.2%, 7.9%; 82.3% in trial participants vs. 77.2% in non-participants). Trial participants had similar risk of parity before age 20 compared to non-trial participants (aRD 2.3%, 95% CI: -0.8%, 5.5%). Conclusions: Trial participation did not appear to influence early parity but did increase educational attainment. Our results are compatible with an explanation of Hawthorne effects from trial participation on schooling behaviors that were small but observable even four years after the end of the trial.