A rigorous statistical Coulomb interaction simulator that can take into account mask electrons was developed. In electron-beam (EB) projection lithography, many electrons incident to an absorber or a scatterer pass through the EB mask. These mask electrons, however, have not been considered in a simulation of Coulomb interaction. In a novel simulation approach, an assessment has been made of the effects of mask electrons on image quality and flare due to electron-electron interactions. The statistical electron-electron interactions from mask electrons under various conditions are simulated by injecting them into a simulation domain and using the fast multipole accelerated method to rigorously follow all interactions as a function of time. It was found that mask electrons contribute to the trajectory displacement effect and increase the number of deviated electrons in the pattern area by four to five times when the accelerating voltage is 125 kV, the beam current density is 0.1 A/cm(2), the mask thickness is 10 mu m, and the electron transmittance is 95%.