The introduction of a new zoning ordinance to Chicago in 1923 offers a natural experiment that allows us to determine the effects of zoning on relative land-value growth rates. Policymakers claimed at the time that land-use zoning would raise aggregate land values by reducing negative externalities associated with mixed land use. After controlling for initial land use and the endogeneity of zoning decisions, we find that residential zoning led to higher land value growth rates than commercial zoning.