OBJECTIVES To examine the relationship between primary diagnoses and mobility impairment and recovery among hospitalized older adults. DESIGN Prospective cohort study. SETTING UF Health Shands Hospital, an 852-bed level I trauma center located in Gainesville, Florida. PARTICIPANTS A total of 18,551 older adults (≥65 years) with 29,148 hospitalizations between January 2009 and April 2014. MEASUREMENTS Incident and discharge mobility impairment and recovery were assessed using the Braden activity subscale score that was recorded by the nursing staff at every shift change: approximately 3 times per day. Primary diagnosis ICD-9 codes were used as predictors and recategorized by using the Agency for Health Care Research and Quality Clinical Classification Software. RESULTS Of the 15,498 hospital records in which the patient was initially observed to "walk frequently," 3186 (20.6%) developed incident mobility impairment (chair-fast or bedfast). Primary diagnoses with a surgical or invasive procedure were the most prevalent (77.2%) among the hospital observations with incident mobility impairment; otherwise, primary diagnoses without surgery were much more associated with discharge mobility impairment (59%). The highest incidence of mobility impairment occurred in patients with heart valve disorders and aortic and peripheral/visceral artery aneurysms (6.24 and 6.05 events per 30 person-days, respectively); septicemia showed the highest incidence rate for mobility limitation at discharge (0.94 events per 30 person-days). Mobility impairment was observed in 13,650 (46.8% of total) records at admission and 5930 (43.44%) were observed to recover to a state of walking occasionally or frequently. Osteoarthritis and cancer of gastrointestinal organs/peritoneum had the highest incidence rate for mobility recovery (7.68 and 5.63 events per 30 person-days respectively). CONCLUSIONS Approximately 1 of 5 patients who were mobile at admission became significantly impaired during hospitalization. However, approximately half (43.4%) of patients observed to have mobility impairment at admission recovered during hospitalization. Conditions most associated with mobility impairment and recovery are varied, but older patients hospitalized for septicemia and cardiovascular diseases with surgery (heart valve disorders and aortic/peripheral/visceral artery aneurysms) appear to be at most risk for incident mobility impairment that did not recover at discharge.