OBJECTIVES: To estimate the prevalence of different levels of mobility in a hospitalized older cohort, to measure the degree and rate of adverse outcomes associated with different mobility levels, and to examine the physician activity orders and documented reasons for bedrest in the lowest mobility group. DESIGN: A prospective cohort study. SETTING: An 800-bed university teaching hospital. PARTICIPANTS: Four hundred ninety-eight hospitalized medical patients, aged 70 and older. MEASUREMENTS: Using average mobility level, scored from 0 to 12, the low-mobility group was defined as having a score of 4 or less, intermediate as a score of higher than 4 to 8, and high as higher than 8. Outcomes were functional decline, new institutionalization, death, and death or new institutionalization. RESULTS: Low and intermediate levels of mobility were common, accounting for 80 (16%) and 157 (32%) study patients, respectively. Overall, any activity of daily living (ADL) decline occurred in 29%, new institutionalization in 13%, death in 7%, and death or new institutionalization in 22% of patients in this cohort. When compared with the high mobility group, the low and intermediate groups were associated with the adverse outcomes in a graded fashion, even after controlling for multiple confounders. The low-mobility group had an adjusted odds ratio (OR) of 5.6 (95% confidence interval (CI) = 2.9-11.0) for ADL decline, 6.0 (95% CI = 2.5-14.8) for new institutionalization, 34.3 (95% CI = 6.3-185.9) for death, and 7.2 (95% CI = 3.6-14.4) for death or new institutionalization. The intermediate group had adjusted ORs of 2.5 (95% CI = 1.5-4.1), 2.9 (95% CI = 1.4-6.0), 10.1 (95% CI = 1.9-52.9), and 3.3 (95% CI = 1.8-5.9) for ADL decline, new institutionalization, death, and death or new institutionalization, respectively. Bedrest was ordered at some point during hospitalization in 165 (33%) patients. For most patients, mobility was limited involuntarily (bedrest orders), and almost 60% of bedrest episodes in the lowest mobility group had no documented medical indication. CONCLUSION: Low mobility and bedrest are common in hospitalized older patients and are important predictors of adverse outcomes. This study demonstrated that the adverse outcomes associated with low mobility and bedrest may be viewed as iatrogenic events leading to complications, such as functional decline.