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.