An understanding of why parents provide consent for adolescent participation in research on sensitive topics can inform and improve the ethical conduct and review of such research. As part of a longitudinal study of sexually transmitted infections (STIs) in lower-income adolescents, we asked 134 parents why they permitted their daughter to participate, analyzing responses using qualitative methods. Over half described participation benefits, providing reasons such as the study being generally good for their daughters, sex education, someone to talk to, and STI testing. Other reasons included positive interactions and familiarity with research and clinical staff, friend or family member participation, and adolescent autonomy in making the decision to participate. If parents perceived their daughter to be “at risk” in some way, such as for STI or pregnancy, they were more likely to cite participation benefits. These data can be used to make such research more sensitive to family and community needs.