The process of development of a Dutch smoking prevention project is described. An essential feature of the project is the combination and interaction of qualitative and quantitative research methods. It is advocated that each method has its own contribution and can be considered as a separate methodology contributing to social science in general and health education research in particular. Combining the two approaches in a spiral approach will result in a synergistic effect, because of the interaction of both approaches. The results of both methods suggest that qualitative methods enhanced the generation of ideas and theories. Qualitative methods were used to formulate ideas for improving quantitative data gathering, analyzing and comparing ideas with respect to program development, and for testing the internal validity of a quantitative design. The quantitative method enabled testing of results in different groups and detecting detailed differences. It also provided information that one of the assumptions of the program, the development of a teacher independent program, was not completely realized. The major advantage of using both methods is that this provides feedback between assumptions and data, thus enhancing comparison of results and critical reflection during the whole project.