The paper presents evidence supporting the existence of a stable money demand relationship for Germany plus a core group of countries—France, Belgium, Denmark, Luxembourg and the Netherlands—that have not realigned their parities against the deutschmark since at least 1987. The predictive power of the core ERM aggregate for French and German inflation is also examined; it is shown that the ERM aggregate is a better predictor of German inflation than the German monetary aggregate alone. Thus, the ERM money supply is a useful indicator for German monetary policy, even if the latter only focuses on achieving domestic inflation targets.