We analyze the causal effects of real and nominal macroeconomic uncertainty on inflation and output growth and examine whether these effects vary with the level of inflation and location on the business cycle. Employing a bivariate Smooth Transition VAR GARCH-M model for the G7 countries during the period 1957-2009, we find strong nonlinearities in these effects. First, uncertainty regarding the output growth rate is related with a higher average growth rate mostly in the low-growth regime, supporting the theory of "creative destruction". Second, higher inflation uncertainty induces lower growth rates, increasingly so at the high-inflation regime. Third, real and nominal uncertainties have mixed effects on average inflation. Nevertheless, there is a trend in favour of the Cukierman-Meltzer hypothesis in the high-inflation regime. Our results can be viewed as offering an explanation for the often mixed and ambiguous findings in the literature.