New Keynesian models of monetary policy downplay the role of monetary aggregates, in the sense that the level of output, prices, and interest rates can be determined without knowledge of the quantity of money. This paper evaluates the empirical validity of this prediction by studying the effects of shocks to monetary aggregates using a vector autoregression (VAR). Shocks to monetary aggregates are identified by the restrictions suggested by New Keynesian monetary models. Contrary to the theoretical predictions, shocks to broad monetary aggregates have substantial and persistent effects on output, prices and interest rates. (C) 2009 Elsevier B.V. All rights reserved.