In most cities, it is becoming evident that the increase in energy consumption is causing environmental problems, including a temperature rise in the urban atmosphere (an urban heat island) and air pollution. The present paper reports on the results of a three-dimensional computer simulation of the urban heat island in the Tokyo metropolitan area, as well as moisture migration. 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. According to a recent computer simulation for urban warming in the Tokyo metro area in 2031, the maximum ambient temperature in the evening (18 : 00) at Otemachi will exceed 42°C. In contrast to the interior thermal comfort in residential and office buildings, the urban outdoor comfort is strongly affected by intense thermal radiation coming from the surface of the structures, as well as solar radiation. Motivated by the above facts, we propose a new standard effective temperature index (USET^*), which is applicable to urban comfort. By using this comfort index, assessment of the urban environment is made for both presentday Tokyo and Tokyo around 2030. Furthermore, it is suggested that a comprehensive urban environmental index (UEI). which includes pollutants and ultraviolet rays, is adopted for the future urban environment.