A minority of adolescent and young adult cancer patients (AYA) live with an uncertain or poor prognosis (UPCP). Caring for a young, advanced cancer patient can lead to caregiver burden. This study aims to provide insight into burden on informal caregivers of AYA cancer patients with UPCP. In-depth, semistructured interviews were conducted with parents (n = 12), siblings (n = 7), friends (n = 7), and partners (n = 13). Thematic analysis was performed to derive themes from the data. Participants reported sleeping problems and stress. They struggle with uncertainty, fear, loss, and negative emotions. Family life is altered due to solely taking care of the children, but also the AYA. Contact with friends and family is changed. The relationship to the AYA can shift positively (e.g., becoming closer) or negatively (e.g., more conflict or no longer being attracted). Participants were under pressure, having to take on many responsibilities and multiple roles. In the financial domain, they report less income and often must continue working. A high amount of caregiver burden is experienced among informal caregivers of AYAs with UPCP. Yet only part of the impact appears to be age specific. Specific, age-adjusted interventions can be developed to lower the burden.