Purpose: The prevalence of intimacy and substance use among adolescents and young adults during cancer therapy has not been well described. Methods: The "Resilience in Adolescents and Young Adults with Cancer" study was a prospective, multicenter, mixed-methods cohort study. English-speaking patients 14-25 years old with newly diagnosed cancer were invited to complete a comprehensive survey at the time of enrollment (T1) and 3-6 months later (T2). Intimate relationships and health behaviors were assessed with questions adapted from the Guidelines for Adolescent Preventative Services assessment. Descriptive statistics characterized the prevalence of sexual and substance-related behaviors at each time point. Results: Of 42 eligible and enrolled participants, 35 (83%) and 25 (59%) completed T1 and T2 surveys, respectively. Their mean age was 17.6 years (standard deviation 2.3), 57% were male, and the most common diagnoses were sarcoma and acute leukemia. Over a third of participants reported dating at each time point; 26% were sexually active at T1, and 32% at T2. Of those endorsing sexual activity, fewer than half reported consistent birth control or condom use and 4 reported their first sexual intercourse during our observation. In addition, 46% (T1) and 44% (T2) reported alcohol use and 23% (T1) and 26% (T2) reported illicit drug use. Despite these activities, fewer than 10% endorsed a worry or need to discuss these behaviors with oncology providers. Conclusions: Intimacy and substance use among adolescents and young adults are common during cancer therapy. Clinical and research implications include the identification of optimal communication and patient-centered supports. (C) 2016 Society for Adolescent Health and Medicine. All rights reserved.