The employment share of initially more dense metro areas declined and those of initially less dense metro areas rose. Motivated by this finding, the authors develop a system-of-cities model in which increase in aggregate metropolitan employment causes employment to shift in favor of less dense metro areas because congestion costs increase more rapidly for the initially more dense metro areas. A calibrated version of the model shows that the more-than-twofold increase in employment experienced by MSAs during the postwar period was indeed a powerful force favoring deconcentration.