A raising number of patients with osteosynthesis of the proximal femur and additional dementia will be seen in hospitals in the future due to demographic changes. There is an ongoing discussion, if and to what extent cognitive abilities do influence functional outcome in geriatric rehabilitation. We therefore compared 250 patients with osteosynthesis of the proximal femur of whom one half had additional dementia, by a matched-pair analysis for the improvement of mobility assessed by the mobility items of the Barthel Index and the Tinetti mobility index. Dementia was an important cofactor for the success of geriatric rehabilitation. Patients with additional dementia reached lower mobility scores at discharge. Also, patients with dementia had significantly more in-hospital falls. Furthermore, the study revealed that demented patients received less individual and group therapy per hospital day. The study underlines the need for specialized wards treating demented patients with additional illnesses.