This paper studies the growth and welfare effects of macroprudential regulation in an overlapping generations model of endogenous growth with banking and agency costs. Indivisible investment projects combine with informational imperfections to create a double moral hazard problem à la Holmström-Tirole and a role for bank monitoring. When the optimal monitoring intensity is endogenously determined, an increase in the required reserve ratio (motivated by systemic risk considerations) has conflicting effects on investment and growth. On the one hand, requiring banks to put away a fraction of the deposits that they receive reduces the supply of loanable funds. On the other, a higher required ratio raises incentives to save and mitigates banks' incentives to monitor, thereby lowering monitoring costs and freeing up resources to increase lending. In addition, it may mitigate the systemic risk externality associated with excessive leverage. This trade-off can be internalized by choosing the required reserve ratio that maximizes growth and welfare. However, the risk of disintermediation means that in practice financial supervision may also need to be strengthened, and the perimeter of regulation broadened, if the optimal ratio is relatively high.