Postwar U. S. time series for money, interest rates, prices, and GNP are characterized by a multivariate process driven by four exogenous disturbances. Those disturbances are identified so that they can be interpreted as the four main sources of fluctuations found in the IS-LM-Phillips curve model: money supply, money demand, IS, and aggregate supply shocks. The dynamic properties of the estimated model are analyzed and shown to match most of the stylized predictions of the model. The estimated decomposition is also used to measure the relative importance of each shock, to interpret some macroeconomic episodes, and to study sources of permanent shocks to nominal variables.