In this paper, the human exposure to the electromagnetic field radiated by a cellular base station antenna operating around 900 MHz in an urban environment has been analyzed. A hybrid ray-tracing/FDTD method has been used to evaluate the incident field and the power absorbed in an exposed subject in the presence of reflecting walls. The base station antenna has been characterized by means of its radiation pattern, evaluated with an FDTD analysis of a typical panel antenna. Three particular situations for a rooftop mounted antenna have been considered. In all the examined cases the obtained results, in terms of incident field and absorbed power, are below the most recognized safety standard levels. The importance of an accurate modeling of the environment in which the exposure takes place has been evidenced