Objective: Studies of prognostication in advanced cancer use a wide range of outcomes and outcome measures, making it difficult to compare these studies and their findings. Core Outcome Sets facilitate comparability and standardisation between studies and would benefit future prognostic research. This qualitative study is the second step in developing such a Core Outcome Set, with the aim to explore the perceptions and experiences of patients with advanced cancer, informal caregivers, and clinicians regarding the potential outcomes of prognostication. Methods: A qualitative study using semi-structured interviews and framework analysis, informed by the Core Outcome Measures for Effectiveness Trials (COMET) taxonomy. We conducted semi-structured interviews with patients living with advanced cancer (n=8), informal caregivers (n=10), and clinicians (n=10) recruited from palliative care services across three sites in London, United Kingdom. Interviews were conducted in-person, via telephone, or video conferencing. Results: We identified 33 outcomes, 16 of which were not previously reported in the literature. We grouped outcomes into 10 domains, using a modified COMET taxonomy: 1) mortality/survival; 2) general physiological/clinical outcomes; 3) psychiatric outcomes; 4) spiritual/religious/existential functioning/wellbeing; 5) emotional functioning/wellbeing; 6) social functioning; 7) delivery of care; 8) perceived health status; 9) personal circumstances; 10) societal/carer burden. Conclusions: This study offers valuable insights into outcomes significant to key stakeholders, underscoring the need for a patient-centred approach in research and clinical practice in prognostication in advanced cancer. These outcomes will play a key role in the development of a Core Outcome Set to assess the impact of prognostication in advanced cancer.