Study Objective: Few studies have explored what specific outcome measures contained in assessment tools for period and pelvic pain are most relevant to adolescents. Co-design is a valuable method of ensuring input from those with lived experience. The Longitudinal Study of Teenagers with Endometriosis Periods and Pelvic Pain in Australia (LongSTEPPP) Co-Design Periods Survey comprised an anonymous online survey of adolescents' experience of menstruation to inform patient-reported outcome measures for the larger 5-year project., Methods: Adolescents aged 12-18 years whose periods had commenced at least 3 months previously and with demonstrated capacity to consent were invited to participate in an online survey. Recruitment was primarily via social media channels., Results: Of the 1811 adolescents who participated, 85% reported that periods had a "moderate" or greater impact on their life. Pain (90.7%), heavy flow (56.2%), and worry about leakage (49%) were common reasons for missed activities. Menstrual symptoms were wide-ranging and included cramping, nausea, poor energy, and impacts on mood. When asked where adolescents sought assistance with their periods, 39.8% had seen their general practitioner, 21.3% their school nurse, and almost 1 in 10 had consulted a mental health practitioner (9.3%). To manage menstrual symptoms, heat packs (66.0%), over-the-counter medications (55.8%), and prescription medications (28.6%) were used., Conclusion: We found a lack of menstrual health awareness in adolescents. Periods had a significant effect on their lives, and adolescents commonly missed activities. In managing menstruation, a wide range of practitioners were consulted. Nearly a third were prescribed medication to manage their periods. These findings have directed the longitudinal study as to how best to capture outcome measures that reflect the impact of periods on adolescents., Competing Interests: Conflicts of Interest The authors have no conflicts of interest to declare., (Copyright © 2023. Published by Elsevier Inc.)