Objectives: Older patients are typically underrepresented in clinical trials despite representing a major proportion of the patient population. We aim to describe the feasibility of performing body composition measures, physical function measures, and patient-reported questionnaires within the first 24 hours of admission in a large sample of older acutely admitted medical patients. In addition, we aim to characterize patients with missing measurements. Design: Secondary analyses of cross-sectional data from a cohort study. Setting and Participants: A total of 1071 acutely admitted patients aged ≥65 years from the acute medical ward at Bispebjerg Hospital, were enrolled within the first 24 hours of hospitalization. Methods: Body composition was investigated using direct segmental multifrequency bioelectrical impedance analyses (DSM-BIA) and physical function was assessed using hand grip strength (HGS) and the 30-second sit-to-stand test (STS). The orientation-memory-concentration test (OMC) was used to evaluate the prevalence of cognitive impairments within 24 hours of hospitalization, and the OMC in conjunction with the Strength, Assistance walking, Rise from a chair, Climb stairs, and Falls questionnaire (SARC-F) was used to assess the feasibility of patient-reported outcomes (PROs). Results: Mean age was 78.8 ± 7.8 years (53.0% female). HGS was performed in 96.2% of the enrolled patients, whereas the PRO, 30-second STS, and DSM-BIA were performed in 91.2%, 69.2%, and 59.8% of patients, respectively. The main barrier for performing the 30-second STS and body composition measurements was an inability to mobilize the patient from the hospital bed. Conclusions and Implications: The assessment of HGS and PROs show excellent feasibility in clinical research including older patients, even when the patients are enrolled and tested within 24 hours of an acute admission. Assessments of DSM-BIA and the 30-second STS show good feasibility but are less feasible in immobile patients often presenting as more frail, weaker, and cognitively impaired.