Objectives: To examine the proportion of time spent in three levels of mobility (lying, sitting, and standing or walking) by a cohort of hospitalized older veterans as measured by validated wireless accelerometers. Design: A prospective, observational cohort study. Setting: One hundred fifty-bed Department of Veterans Affairs hospital. Participants: Forty-five hospitalized medical patients, aged 65 and older who were not delirious, did not have dementia, and were able to walk in the 2 weeks before admission were eligible. Measurements: Wireless accelerometers were attached to the thigh and ankle of patients for the first 7 days after admission or until hospital discharge, whichever came first. The mean proportion of time spent lying, sitting, and standing or walking was determined for each hour after hospital admission using a previously validated algorithm. Results: Forty-five male patients (mean age 74.2) with a mean length of stay of 5.1 days generated 2,592 one-hour periods of data. A baseline functional assessment indicated that 35 (77.8%) study patients were willing and able to walk a short distance independently. No patient remained in bed the entire measured hospital stay, but on average, 83% of the measured hospital stay was spent lying in bed. The average amount of time that any one individual spent standing or walking ranged from a low of 0.2% to a high of 21%, with a median of 3%, or 43 minutes per day. Conclusion: This is the first study to continuously monitor mobility levels early during a hospital stay. On average, older hospitalized patients spent most of their time lying in bed, despite an ability to walk independently. © 2009, The American Geriatrics Society.