Background: Limited attention has been paid to adolescents and young adults' (AYA's) experiences in the aftermath of a cancer diagnosis, despite this being a time when potentially life-changing decisions are made. We explored AYA's and caregivers' experiences of, and views about, making treatment and trial participation decisions following a cancer diagnosis, in order to understand, and help facilitate, informed treatment decision-making in this age group. Methods: Interviews were undertaken with 18 AYA diagnosed, or re-diagnosed, with cancer when aged 16-24 years, and 15 parents/caregivers. Analysis focused on the identification and description of explanatory themes. Results: Most AYA described being extremely unwell by the time of diagnosis and, consequently, experiencing difficulties processing the news. Distress and acceleration in clinical activity following diagnosis could further impede the absorption of treatment-relevant information. After referral to a specialist cancer unit, many AYA described quickly transitioning to a calm and pragmatic mind-set, and wanting to commence treatment at the earliest opportunity. Most reported seeing information about short-term side-effects of treatment as having limited relevance to their recovery-focused outlook at that time. AYA seldom indicated wanting to make choices about front-line treatment, with most preferring to defer decisions to health professionals. Even when charged with decisions about trial participation, AYA reported welcoming a strong health professional steer. Parents/caregivers attempted to compensate for AYA's limited engagement with treatment-relevant information. However, in seeking to ensure AYA received the best treatment, these individuals had conflicting priorities and information needs.