ObjectiveTo identify core domains for research studies of physical activity and sedentary behavior during hospitalization for older adults with an acute medical illness. DesignA 4-Round Delphi consensus process. Round 1 invited responses to open-ended questions to generate items for the core domains research. In rounds 2-4, participants were invited to use a Likert scale (1-9) to rate the importance of each core domain for research studies of physical activity and/or sedentary behavior in hospitalized older adults with an acute medical illness. SettingOnline surveys. ParticipantsA total of 49 participants were invited to each round (international researchers, clinicians, policy makers and patients). Response rates across rounds 1-4 were 94%, 88%, 83% and 81%, respectively. InterventionsNone. Main Outcome MeasuresConsensus was defined a priori as ≥70% of respondents rating an item as “critical” (score≥7) and ≤15% of respondents rating an item as “not important” (score≤3). ResultsIn round 2, a total of 9 of 25 core domains reached consensus agreement (physical functioning, general, role functioning, emotional functioning, global quality of life, hospital, psychiatric, cognitive functioning, carer burden). In round 3, an additional 8 reached consensus (adverse events, perceived health status, musculoskeletal, social functioning, vascular, cardiac, mortality, economic). Round 4 participants provided further review and a final rating of all 17 core domains that met consensus in previous rounds. Four core domains were rated as “critically important” to evaluate: physical functioning, social functioning, emotional functioning, and hospital outcomes. ConclusionsThis preliminary work provides international and expert consensus-based core domains for development toward a core-outcome set for research, with the ultimate goal of fostering consistency in outcomes and reporting to accelerate research on effective strategies to address physical activity and/or sedentary behavior in older adults while hospitalized.