Forecasts for the NOx concentration levels in the Greater Athens area in 2004 are compared to the corresponding figures for 1990. Simulations are performed for two meteorological cases using four different dispersion models. Two different emission inventories are employed in the simulations. The first is based on the conditions for the year 1990, while the second is the reference scenario for the year 2004, taking into account all major public works under construction. In order to ensure the validity of the individual models, simulation results are compared with available measurements for the year 1990. All models show an overprediction of the maximum NOx concentrations, but in general the simulation results show satisfactory agreement with the observations. Excellent agreement is found between the results of all models with regard to the distribution of the 50 maximum hourly NOx concentrations. Reductions of the peak NOx levels of the order of 35% are forecast by all models between 1990 and the reference scenario for 2004.