BACKGROUND: Inactivity during hospitalization in older medical patients may lead to functional decline. This study quantified 24-hour mobility, validated the accelerometers used, and assessed the daily level of basic mobility in acutely admitted older medical patients during their hospitalization. METHODS: This is a prospective cohort study in older medical patients able to walk independently (ambulatory patients) and those not able to walk independently (nonambulatory patients) on admission. The 24-hour mobility level during hospitalization was assessed by measuring the time in lying, sitting, and standing and/or walking, by two accelerometers. Basic mobility was quantified within 48 hours of admission and repeated daily throughout hospitalization. RESULTS: Forty-three ambulatory patients and six nonambulatory patients were included. The ambulatory patients tended to be hospitalized for fewer days than the nonambulatory patients (7 vs 16, p = .13). The ambulatory patients were lying median 17 hours, (interquartile range [IQR]: 14.4-19.1), sitting 5.1 hours (IQR: 2.9-7.1), and standing and/or walking 1.1 hours (IQR: 0.6-1.7) per day. On days with independency in basic mobility, the ambulatory patients were lying 4.1 hours less compared with days with dependency in basic mobility (p <.0001), sitting 2.4 hours more (p = .0004), and standing 0.9 hours more (p <.0001). The algorithm identification for lying, sitting, and standing and/or walking of the accelerometers, corresponded by 89%-100% with positions performed by older medical patients. CONCLUSIONS: Older acutely hospitalized medical patients with walking ability spent 17h/d of their in-hospital time in bed, and the level of in-hospital mobility seemed to depend on the patients' level of basic mobility. The accelerometers were valid in assessing mobility in older medical patients.