BACKGROUND: Research studies are essential to improving healthcare. However, many fail to recruit their planned number of participants. There are many interventions that researchers try to improve recruitment. Finding which ones are effective would be of benefit to the research community and society. OBJECTIVES: To quantify the effects of strategies to improve participation in research studies. SEARCH STRATEGY: We aimed to find all randomized and quasi-randomized controlled trials of strategies to improve recruitment to research studies. We searched nine electronic databases and manually searched the reference lists of relevant trials. We assessed the eligibility of each trial using pre-defined criteria. SELECTION CRITERIA: Randomized and quasi-randomized controlled trials of methods to increase recruitment in research studies. This includes non-healthcare studies. Studies that required only questionnaire completion were excluded. DATA COLLECTION AND ANALYSIS: We extracted data on the method evaluated, nature of the population, nature of the study to be recruited into, randomisation or quasi-randomisation method, allocation concealment, numbers and proportions in each arm. We used risk ratios and their 95% confidence intervals to describe the effects in individual trials, and assessed heterogeneity of these ratios between trials. MAIN RESULTS: We identified 15 eligible trials, including a total of 33,719 participants. All strategies were aimed at participants for healthcare studies. No strategies were identified at the level of researcher collaborators or ethics committees. Because of heterogeneity between trials and within strategies, the results were not synthesised. Trials of monetary incentives, an additional questionnaire at invitation and treatment information on the consent form demonstrated benefit; these specific interventions from individual trials are not easily generalizable. AUTHORS' CONCLUSIONS: On this evidence, it is not possible to predict the effect most interventions will have on recruitment. Funders and researchers should ensure that the evaluation of recruitment strategies are incorporated into research studies.