["The exclusion of older participants from clinical research is common and limits the generalisation of research findings. We aimed to assess the current potential for older patients to participate in Parkinson's disease (PD) research.", 'We performed a systematic analysis of data extracted from the World Health Organization Clinical Trials Registry Platform regarding 206 actively recruiting PD research studies. Data regarding study variables and exclusion on the grounds of an upper age limit was extracted from each registry entry and subsequently used for statistical analysis.', 'Exclusion by arbitrary upper age limit is common, with 101 (49%) of studies excluding participants by age and with a mean upper age limit for exclusion of 79.3 years (range 64-95 years). Exclusion by age was significantly more common in studies with an estimated enrollment of fewer than 100 participants; OR 1.92 (95%CI 1.13-3.42) P = 0.018. Rates of exclusion by age were not significantly influenced by study subject, study location, source of funding, study duration or number of centres.'] Are older participants frequently excluded from Parkinson 's disease research?