Background: Inactivity and bedrest are frequent among older patients during hospitalization and are associated with functional decline. No longitudinal objective evidence exists on how patients' diurnal profile for physical activity changes during acute hospitalization and what happens when the patient is discharged to own home. By the use of accelerometers, this study aims to describe and compare the distribution of physical activity and sedentary behavior in acutely hospitalized older patients during hospitalization and after discharge. Methods: The study population consisted of 80 patients (+65 years) from the STAND-Cph trial, who were admitted with acute medical illness and were able to stand. Physical activity and sedentary behavior were measured as number of steps, uptime (walking/standing) and sedentary behavior (lying/sitting) per hour with an activity monitor worn on the thigh (activPAL3, PAL Technologies Ltd). The patients wore the monitor for three periods of one week: during hospitalization, after discharge, and four weeks after discharge. Results: The patients’ median age was 80 years [IQR: 75-88], 68% of the patients were female and The median DEMMI was 57 [IQR: 48-67]. The daily median uptime was 1.7 h [IQR: 1-2.8] during hospitalization, 4.0 h [IQR: 2.7-5.4] after discharge and 4.0 h [IQR: 2.8-5.8] four weeks after discharge. The median number of steps was 728 [IQR: 176-2089], 2207 [IQR: 1433-3148], and 2622 [IQR: 1714-3865], respectively. During hospitalization, a small peak was observed both for number of steps and uptime between 9-11 AM. However, the diurnal profile had no notable variation. At discharge and four weeks after discharge, the median of physical activity peaked 2-3 times and had mostly the same diurnal profile. Conclusion: Older hospitalized patients spend most of their time engaged in sedentary behavior, where their main activity peak is in the morning between 9-11 AM. The uptime doubles and the number of steps triples after discharge, where peaks of activity are seen throughout the day. This indicates that daily routines are disrupted during hospitalization and that older patients have the potential to be more physically active during hospitalization. This emphasizes the need for interventions that encourage physical activity throughout the day during hospitalization.