In general, regional economic analysis of trade unions is particularly directed at the effects of labour unions, whereas the conditions and causal aspects which influence the presence of labour unions in regions are studied far less. In this paper an attempt is made to fill part of this gap in knowledge by means of an analysis of the causes of the regional differentiation in labour union membership in the Netherlands. A theoretical survey into the causes of variation in regional membership is reported on. Subsequently, the effects of the potential causes of this regional differentiation are analysed empirically. First, it was found that several potentially important aspects do not explain the regional variation in membership. Second, those aspects that were relevant can be reduced to two common factors. Third, it can be concluded that the rather low level of explanation of the regional differentiation suggests that present economic locational analysis should be supplemented by an approach in which the economic historical context is taken into account too. Regional differentiation in trade union membership can only fully be understood from a regional economic historical approach.