To provide accounting information for management control purposes, two fundamental options exist: (a) The financial records can be used as a database for management accounting (integrated accounting system design), or (b) the management accounting system used by controllers can be based upon a so-called third set of books besides the financial and tax accounting records. Whereas the latter approach had been typical for firms in German-speaking countries until the 1980s, since then an increasing number of them has been changing towards a (partially) integrated accounting system design.In accounting literature, a debate is still on whether this change is for the better or for the worse, especially as from a normative theory perspective no dominant standard exists that provides appropriate information for any given accounting problem which would support a separate accounting system design. Our paper adds to this debate by empirically analyzing the impact of an increasingly integrated accounting system design on controllership effectiveness. Our analysis is distinctive for two reasons. First, we usea dyadic research design surveying both controllers and representatives of general management which allows us to include management s perspective into our analysis in a valid fashion. Second, we do not restrict our model to the technical or supply-side characteristics of management accounting system design, but expand our approach to the user perspective of management accounting information as a financial language.By using structural equation modeling for a sample of 149 dyads surveyed from the German Top-1500 firms in 2007, we identify no statistically significant direct effect of the technical accounting system design, but a fully mediating influence of a unified financial language on controllership effectiveness. Our results imply amongst others that consistency of management accounting with financial reporting, which results from an integrated accounting system design, is an important property of management accounting information. Especially in the context of a financially oriented management control system, controllers therefore should take special care to align their internal reporting with the financial accounting reports provided to investors.