Objective : To describe medicines regimens used by older people living in residential aged care facilities (RACFs). Materials and methods : This cross-sectional study presents baseline data from a randomised controlled trial in seventeen Australian RACFs that recruited residents aged 65 years and older at the participating facilities. The main outcome measures were to evaluation of medicines utilisation, including the number of medicines, medicine regimen complexity, potential under-prescribing and high-risk prescribing (prescribing cascades, anticholinergic or sedative medicines or other potentially inappropriate medicines) with data analysed descriptively. Results : Medicines regimens were analysed for 303 residents (76% female) with a mean age of 85.0 ± 7.5 years, of whom the majority were living with dementia (72%). Residents were prescribed an average of 10.3 ± 4.5 regular medicines daily. Most participants (85%) had highly complex regimens. Most residents (92%) were exposed to polypharmacy (five or more medicines). Nearly all, 302 (98%) residents had at least one marker of potentially suboptimal prescribing. At least one instance of potential under-prescribing was identified in 86% of residents. At least one instance of high-risk prescribing was identified in 81% of residents including 16% of participants with at least one potential prescribing cascade. Conclusion(s) : Potentially suboptimal prescribing affected almost all residents in this study, and most had highly complex medicines regimens. If generalisable, these findings indicate most older people in RACFs may be at risk of medicines-related harm from suboptimal prescribing, in addition to the burden of administration of complex medicines regimens for facility staff and residents.