We provide empirical evidence of the dynamics of city size distribution for the whole of the twentieth century in U.S. cities and metropolitan areas. We focus our analysis on the new cities that were created during the period of analysis. The main contribution of this paper, therefore, is the parametric and nonparametric analysis of the population growth experienced by these new-born cities. Our results enable us to confirm that, when cities appear, they grow very rapidly and, as the decades pass, their growth slows or even falls into decline. This is consistent with the theoretical framework regarding mean reversion (convergence) in the steady state and with the theories of sequential city growth.