This article argues that, despite relatively good integration policies and stable, high and cross-sectoral national trade union membership, irregular immigration and irregular work in Belgium threaten workers' solidarity and trade union strength. The diffusion of 'twilight' workers in many companies and sectors has obliged the trade unions to call for regularisation to restore the workers' front and re-establish workers' cohesion. Although the trade unions have supported a number of actions in support of people without documents (such as hunger strikes in churches) it is not easy to convince unionised workers to accept the new 'comrades' and to fight racism in the workplace. Moreover, there are other difficulties, including the discussion among policy-makers of an EU 'Blue Card' for temporary migration for work. The latter in particular represents a new challenge for the Belgian trade unions, who traditionally have supported family reunification and long-term residence of migrants.