BackgroundFor valid informed consent, it is crucial that patients or research participants fully understand all that their consent entails. Testing and revising informed consent documents with the assistance of their addressees can improve their understandability. In this study we aimed at further developing a method for testing and improving informed consent documents with regard to readability and test-readers’ understanding and reactions.MethodsWe tested, revised, and retested template informed consent documents for biobank research by means of 11 focus group interviews with members from the documents’ target population. For the analysis of focus group excerpts we used qualitative content analysis. Revisions were made based on focus group feedback in an iterative process.ResultsFocus group participants gave substantial feedback on the original and on the revised version of the tested documents. Revisions included adding and clarifying explanations, including an info-box summarizing the main points of the text and an illustrative graphic.ConclusionOur results indicate positive effects on the tested and revised informed consent documents in regard to general readability and test-readers’ understanding and reactions. Participatory methods for improving informed consent should be more often applied and further evaluated for both, medical interventions and clinical research. Particular conceptual and methodological challenges need to be addressed in the future.Electronic supplementary materialThe online version of this article (10.1186/s12910-017-0232-7) contains supplementary material, which is available to authorized users.