In the 1990s, due to the events in Brazilian economy, workers' unions faced a significant crisis. The union's bases contracted and, at the same time, became more rarefied. However, in 2000, despite the good shape of the economy (at least in its macro context), workers' affiliation to the unions did not advance (at least, not as it might have advanced). Indeed, the 2000s showed a gap between the labour and union dynamics. On the one hand, the first advanced through the creation of millions of new jobs, in the wake of events in Brazilian economy. On the other hand, the second did not advance (or, at least, not as much), because unions reached only a fraction of the occupants of those new jobs. In short, union's bases widened, but rarely thickened. What were the reasons behind this discrepancy between the labour and union dynamics in the 2000s? What could explain the lack of density of union's bases, despite their concomitant expansion? In this article, the objective is to raise some hypotheses, situated in three fields of discussion (the field of the economy, of the demographics/society and of the politics).