The inflation-output relationship in the US for the period of 1955-1990 is examined. Smyth (1992) is replicated and his estimates are subjected to a series of diagnostic tests. The model is shown to satisfy conditions for valid inference and policy analysis. These robustness checks allow the study of out-of-sample consequences of the point estimates for various levels of inflation. One central experimental result is that for inflation rates exceeding 4%, the natural rate of output is reduced to such an extent that it contributes to a reduction in the growth rate of real GNP that is below historical trend.