The paper explores whether and how unions in the post-socialist EU member states have responded to the opportunity of improving their situation, offered by the increased emigration after the recent EU enlargements. Migration influences the labour force composition and unemployment rates, which could facilitate union organizing and bargaining position, and in consequence enhance union legitimacy and bargaining institutions. We adopt an actor-oriented framework to examine union strategies and actions, and we test the above hypotheses in the public healthcare sector largely affected by migration in Slovakia, Poland and Hungary. We argue that variation in union strategies depends mainly on the interplay of union capacities and state strategies. Slovak unions used migration-triggered labour shortages to obtain wage increases and to consolidate existing bargaining channels. In contrast, Polish unions responded to migration-induced labour shortages through industrial action, while Hungarian healthcare unions remained the least active in seizing migration-related opportunities to enhance legitimacy or bargaining institutions.