In this paper, three-dimensional numerical simulations are carried out to evaluate the effect of fouling on an axial compressor stage. A numerical model of the NASA Stage 37, validated in previous papers of the same authors against experimental data available from literature, is used as a case study. The occurrence of fouling is simulated by imposing different spanwise distributions of surface roughness, in order to analyze its effect on compressor performance. To this aim, both the stage performance maps of the fouled compressor and the spanwise distribution of work and losses are analyzed and discussed. Moreover, the definition of an averaged roughness parameter is suggested, to characterize the different roughness distributions.The results show that the drop of overall performance can actually be predicted by means of the numerical model. Moreover, detailed information about fluid-dynamic phenomena can be analyzed on the basis of the actual distribution of surface roughness on rotor blades.