It is nowadays accepted that labour market institutions matter in economic development. However, empirical studies on unions’ effect are not univocal. We provide new insight into the impact of unions on the long-run performance of industrialized economies using an indicator of union centralization. Relying on a recent panel dataset of OECD countries, we estimate a growth equation and show that a high degree of union centralization can be harmful to growth. This effect is not driven by economic channels (such as private investment, human capital and research and development) but appears to be related to a direct effect of unions on the growth process.