This study examined a population-based profile of older adults with acquired brain injury, and their functional outcomes, in in-patient rehabilitation. Older adults aged 65 and older admitted to in-patient rehabilitation from acute care with traumatic brain injury (TBI) (n = 1214) or non-traumatic brain injury (nTBI) (n = 1,530) from 2003/04 to 2009/10 in Ontario were identified. Demographic and clinical characteristics and the total function score from the FIM(™) Instrument were examined. The Discharge Abstract Database and National Rehabilitation Reporting System were used. Results indicated that older adults with TBI had significantly higher total function scores than those with nTBI at admission and at discharge (p < .001). However, both groups made significant (p < .001) and similar gains (p > .05) in total function scores. We conclude that older adults with TBI and nTBI make similar in-patient rehabilitation gains. Lower initial functional ability of nTBI patients on admission and patients' different clinical profiles have implications for clinical care and resources.