In this article, we analyse the mayoral open primaries held from 2004 to 2015. We empirically assess their functioning and their effectiveness, especially in terms of competitiveness and turnout. We also explore the dynamics of diffusion of this instrument of intra-party democracy over time, across parties and across geographical areas. Moreover, we provide an exploratory account of the electoral consequences of mayoral primaries in terms of three dimensions of analysis: the type of municipality in which primaries are held, the features of the primary election contest, and the characteristics of the subsequent mayoral elections. Our results show that local primaries are characterized by two main features: they are fairly institutionalized, and a contagion effect across parties and geographical areas is emerging. In addition, they are quite competitive and our data show that, contrary to popular belief, external and/or more ideologically extreme candidates are less likely to win.