We investigate the impact of the Basel Fundamental Review of the Trading Book (FRTB) on banks’ market risk capital requirements under the internal models approach. To do this, we take a stylized portfolio sensitive to the risk factors affected by the FRTB, representative of a typical trading book. Our assessment spans the period 2007–19. We find that the FRTB will entail sizable increases in the regulatory capital intended to absorb market shocks. These increases originate not only from the change in the risk measure and taking longer liquidity horizons (the latter having a greater impact on portfolios more focused on bonds) but also from the strict limitation of portfolio diversification benefits. Our study should be of interest to bank supervisors and regulators, risk managers and other decision makers within the banking industry.