The literature analysing the efficiency of financial institutions has evolved rapidly over the last 20 years. Most research has focused on the input side, analysing either cost, input technical efficiency or input allocative efficiency, whereas comparatively fewer studies have examined the revenue side. However, both sides are relevant when evaluating banks’ performance. This article explicitly explores how serious it may be to confine the analysis to one side of banks’ activities only, comparing the efficiencies yielded by either minimising costs or maximising revenues. We focus on the Spanish banking sector, which is currently undergoing a profound process of change and restructuring. The application shows how severely biased the analysis is when only a partial efficiency measurement is conducted. It also shows the growing relevance of the issue since the beginning of the financial crisis. © 2016 Asociación Española de Contabilidad y Administración de Empresas (AECA).