BACKGROUND AND AIM: Scientists and scientific institutions are adopting more extensive participatory models, hoping to revisit the existing relationship between science and society. Though citizen science has become more common in environmental monitoring, it is seldom utilized in environmental epidemiology. In the CitieS-Health project, we co-created epidemiological studies with citizens in five European countries. The aim of this paper is to share our experiences and impart methodological insight into the application of co-created citizen science strategies in environmental epidemiology. METHODS: We applied the CitieS-Health framework, involving citizens in all the phases of the studies: identifying research questions, designing research protocols, collecting data, analysing data, interpreting data, formulating conclusions, authoring scientific articles and communicating the results to diverse audiences. These epidemiological studies, conducted in specific areas in Italy, Lithuania, the Netherlands, Slovenia and Spain, covered diverse local environmental issues and health effects ranging from air pollution and mental health to industrial pollution and kidney disease. RESULTS: Together with citizens, we successfully conducted environmental epidemiological studies that generated new scientific knowledge reflecting the concerns and knowledge of citizens. Citizens contributed in all the research activities, including activities beyond formulating the research questions, though the researchers initiated several design discussions and conducted time-consuming and complex tasks (e.g. data analysis, measurement of specific exposures and health outcomes). The challenges we encountered were engaging effectively with citizens throughout the study, harmonizing citizens' knowledge and values with the academics' expertise, managing civic expectations, making complex concepts understandable to citizens and representativeness of participating citizens. The co-created studies were able to empower citizens to address local health concerns by sharing and using scientific knowledge generated from studies. CONCLUSIONS: Integration of co-created citizen science in environmental epidemiology is feasible and has the potential to improve the quality of research whilst promoting civic trust in research and results.