Background: Participatory research (PR) is the co-construction of research through partnerships between researchers and people affected by, and/or responsible for action on, the issues under study. Evaluating the benefits of PR is challenging for a number of reasons: the research topics and study designs are heterogeneous; the extent of collaborative involvement may vary from one project to the next; and partnership activities may generate a complex array of both short- and long-term outcomes. This complexity can manifest as unexpected and unintended outcomes which emerge spontaneously from the collaborative process, but are not well documented in the published literature Methods/Results: We identified, selected, and appraised a large-variety sample of primary studies describing PR partnerships. In each stage, two team members independently coded the literature. We used realist review methodology to synthesize the data, using the PR partnership as the main unit of analysis. From 7,167 abstracts and 591 full-text papers, we distilled for synthesis a final sample of twenty-three PR partnerships described in 276 publications. The link between the collaborative process and unintended/unexpected outcomes will be explained using the theory of partnership synergy. Partnership synergy theory suggests that the productive activity of a collaboration generates outcomes beyond that of which can be achieved by a single party working under similar conditions. Examples of unintended/unexpected outcomes will be described, involving new community infrastructure, systemic changes in community-level health service delivery, and spin-off projects that generate new health research and products. Conclusions: Unintended/unexpected outcomes of PR, while at risk of being overlooked in assessments, are nonetheless important to factor in when examining the outcomes of collaborative processes. This presentation will serve to illuminate this under-documented facet of the academic-community partnership.