Background: Osteoporosis is characterised by low bone mineral density (BMD) leading to an increased risk of fracture. Patients who have sustained a significant traumatic brain injury may have an increased risk of secondary reduced BMD as a result of immobility and other factors. Objectives: To describe BMD in a cohort of patients recovering from traumatic brain injury, and to discuss the implications of the findings for physiotherapy practice. Design: Prospective, observational. Setting: Specialist, residential unit providing care for individuals with brain injury, many with a history of severe challenging behaviour. Participants: Current inpatients (n= 51, 80% male) with the capacity to provide consent, as judged by their responsible clinician. The median age was 41 years (range 20 to 60 years), and the median time since the brain injury was sustained was 22 years (range 4 to 54 years). Methods: Participants' BMD was measured at the radius and tibia using quantitative ultrasound. Various clinical and demographic details were collected. Results: Participants had suboptimal BMD measurements that were generally low for their age and gender. Nine (18%) participants met the criteria for osteopenia measured at the radius, and 26 (51%) participants met criteria for osteoporosis or osteopenia measured at the tibia. Conclusions: Some participants had reduced BMD, putting them at risk of fracture or of developing such risk in the future. This group is at particular risk because they frequently display challenging aggressive behaviours that may be met with responses including proportionate use of manual restraint. Physiotherapists should bear this increased risk in mind when devising exercise programmes assessing risk in neurobehavioural rehabilitation settings.