Bank Holding Companies (BHCs) have been diversifying their businesses increasingly among banking, securities and insurance activities in the recent decades through establishment of Section 20 subsidiaries in earlier years and through formation of financial holding companies after the Gramm–Leach–Bliley Act (GLBA, 1999). This study examines whether BHC diversification is associated with improvement or detriment in its production efficiency. We apply the Data Envelopment Analysis (DEA) to calculate the Malmquist index of productivity, and the total factor productivity change for a sample of BHCs over the period 1997–2007. Two main results are obtained. First, technical efficiency is negatively associated with activity diversification and the effect is primarily driven by BHCs that did not experience diversification through Section 20 subsidiaries. Second, the degree of change in diversification over time is not associated with total factor productivity change but it is negatively associated with technical efficiency change. This latter relationship is also primarily exhibited for BHCs that did not establish Section 20 subsidiaries. It can be concluded that diversification is, on average, associated with lower production efficiency of BHCs, especially for those BHCs without first-mover advantage obtained through Section 20 subsidiaries. This is consistent with findings on diversification discount in the finance literature.