Trade unions will oppose the employment of temporary agency workers if it is a move to replace workers or to curb union power. Alternatively, trade unions may encourage the employment of agency workers if it leads to higher wages for their members. Using British data from 1998 and 2004, we find no evidence for a negative association between trade union activity and the hiring of agency workers. Wages are typically higher in unionized workplaces; however, the trade union premium is lower in the presence of agency workers. Our results suggest that trade unions cannot effectively oppose the hiring of agency workers.