This article explains the ebb and flow in Belgian trade union membership from 1946 to 1995 by replicating the econometric model by Bain and Elsheikhn in which changes in macro-economic variables are highly significant. Since the automatic indexation of wages and the extension of collective labour agreements invite free riding, the relevance of the change in inflation and real wage is quite striking. However, the free riding-effect is slowed down by the institutionalised presence of the trade unions on the work floor. The Ghent system explains the positive impact of the unemployment rate. The model is furthermore improved by the trade union density as a structural variable. The linear form reflects the enforcement effect, while the quadratic form mirrors the saturation effect on the trade union membership. Mainly due to the 'Allgemeinkoalitionsfähighkeit' of the Belgian government system, the impact of left parties on union growth and decline is not significant in a quantitative framework. With only four explanatory variables the model clarifies more than 75% of the fluctuations in Belgian trade union membership.