Thanks to direct access to union databases, this note can answer two new questions in industrial relations: how long union membership lasts and what are the determinants of its duration within an open-shop context. This also allows for conceptualizing union membership as a much more dynamic phenomenon than in previous studies, where it was considered a static condition whose causes or effects were to be investigated. Regression analysis applied to a sample of 29,035 Italian workers highlights that union membership duration is a positive but declining function of age. Furthermore, women, flexible workers, foreign ones, and those working in cities tend to show less attachment to union membership than the other workers.Aim and Structure of the Paper S chnabel (2003), reviewing the literature on union membership, argued that research on this topic should [T]ry to integrate better the different approaches of the various disciplines of social science, it should pay more attention to the process of joining or leaving a union and to union recruitment strategies, and it should attempt to provide a more comprehensive model in which individual workers' optimizing decisions are seen in a wider perspective that pays more attention to the social and institutional background.Thanks to direct access to union databases and by exploiting a data set of 29,035 Italian workers, this contribution meets the second challenge above. In particular, it can answer two questions that the literature on industrial relations could not directly tackle so far: how long union membership lasts and what are the determinants of its duration within an open-shop context.