Rapid progress in industrialization and urbanization has resulted in the concentration of economic growth and social functions in the urban areas. In most urban cities, it has become more and more certain that the increase of energy consumption is causing environmental problems including the temperature rise in the urban atmosphere (urban heat island) and air pollution. The present paper reports on the results of field observation and three-dimensional computer simulation of the urban heat island in the Tokyo metropolitan area. The three-dimensional governing equations for the urban atmospheric boundary layer were formulated by virtue of the vorticity-velocity vector potential method. Particular attention was focused on the representation of a buoyancy term in the equation of motion in the vertical direction, thereby describing the cross-over effect and stratified inversion layer near the ground surface. The vorticity-velocity potential method is superior from the viewpoint of numerical stability and suitable for the simulation of an urban heat island. A survey on the energy consumption was first made in the Tokyo metropolitan area. Next, these data were used for the three-dimensional simulations. In order to evaluate the validity of the present model to simulate the urban heat island, the simulation was carried out by initializing it with the same meteorological conditions as those used for the observation day. The simulation results were then compared with the field observation data for the surface temperature. Quite a good agreement was obtained between the present simulation and the observed data, thereby assuring the present model. Further future prediction of the urban heat island was performed for an energy release rate five times as much as the present rate, which corresponds to the year 2031 if the present consumption rate is maintained until then.