Background: Researchers have noted difficulties in attracting adequate numbers of participants with intellectual disabilities (ID) to their studies. Methods: This study was a review of participation by adults with ID in research conducted in South Eastern Ontario over a 20-year period (1987-2006). Original research studies were identified by local investigators and then reviewed for inclusion and exclusion criteria. The report of each study was then reviewed by three reviewers and key information was extracted. The extent of study participation was calculated using three methods and compared along with key design characteristics. Results: Nine studies met all inclusion/exclusion criteria and provided sufficient data to calculate participation. Among the studies there was a variety of purposes, research designs and recruitment strategies. Using the participant/approached calculation, participation varied between 41.8% and 100%. Higher participation was observed in studies where investigators had direct access to participants, the data collection was non-invasive and consent was required from substitute decision-makers only. There was no clear trend of increasing or decreasing participation over time. Conclusions: Researchers seeking the participation of adults with ID in their studies must incorporate factors influencing participation into study designs to ensure robust results and effective use of research resources.