Recent theoretical models conjecture that the development of the financial sector is essential for economic growth. We investigate this hypothesis from a time-series perspective and find that financial sector GDP is cointegrated for many OECD countries not so much with manufacturing GDP but mostly with manufacturing total factor productivity. Moreover, this relation is in some instances characterized by long-run causality in the sense of Granger and Lin. However, even within this homogeneous group of countries, the variety of results suggests a more complex picture than is apparent from cross-sectional evidence.