Recent destructive earthquakes have confirmed the importance of hazard and vulnerability studies to predict and prevent the impact of large seisms. Most seismic risk analysis considers the buildings as a passive constituent integrated into the vulnerability analysis. This approach neglects the possible contribution of the building vibration to the free field. This article describes an analysis of site-city effects, that is, the seismic interaction of the city with soft soil layers. An analytic method derived from soil-structure interaction studies is described and applied to the Roma Norte zone of Mexico City. The building parameters are derived from detailed studies of the JA and PC buildings. The soil-city system is subjected to the Mexican (Mw 7.3) 14 September 1995 earthquake. The simulated ground surface motions, which incorporate the wave field radiated from the buildings of Roma Norte district, are compared to seismic records from this zone. Analytical procedures reproduce the long time duration and beating of the observed ground motion in Mexico City. Parametric analysis are also performed to identify the predominant factors (e.g., urbanization density, soil-to-city stiffness ratio) that favor the site-city interaction effects. A simple relation is proposed to estimate the expected efficiency of the site-city interaction effects for any city.