OBJECTIVES: A common problem in systematic reviews are incomplete data extraction forms resulting in problems attempting evidence synthesis; we rarely have all the data for the endpoints of interest for all studies, and parameters that inform meta-analysis or connect networks are missing. Increased transparency in clinical trial reporting means this problem is slowly disappearing. From January 2015 the European Medicines Agency (EMA) will publish clinical study reports submitted with marketing-authorisation applications for human medicines.  METHODS: We identified several data sources outside the primary publication. Standard data sources for systematic reviews of interventions include peer-reviewed publications, conference abstracts and clinical trial registries. Clinical study protocols are often published but are not identifiable through searches in online databases, therefore, to find these, systematic reviewers must visit the journal website. Manufacturer submissions to health regulators are also increasingly made available; these give detailed trial descriptions and results presented are more likely to be comprehensive.  RESULTS: In a recent example in Hepatitis-C we utilised several additional data sources in our evidence synthesis. Clinical trial protocols were used to identify definitions of endpoints included and to fulfil aspects of the critical appraisal. Fibrosis stage is an accepted treatment effect modifier in Hepatitis-C; our review therefore collected subgroup data for this. However, this was not readily available in peer-reviewed publications; we thus obtained data from EMA submission documents and UK and German reimbursement submissions. Other examples include a 2013 COPD systematic review which retrieved mortality data from the FDA website for three studies reporting cardiovascular-related death and for one study reporting overall death. CONCLUSIONS: Systematic reviewers should be aware of additional data sources that are publically available. Whilst peer-reviewed data is preferential, incorporation of this grey literature into an evidence synthesis could lead to a more informed overview of clinical efficacy and thereby healthcare decision making.