The case of outdoor to indoor propagation prediction at high elevation angles has scarcely been addressed and no direct relationship between elevation angles and building penetration loss has been reported. High altitude platform stations (HAPS) - a new means of providing international mobile telephony (IMT-2000) - will be operating at high elevation angles. This paper examines the impact of high elevation angles in the propagation mechanism and penetration-into-buildings loss. A 3D ray-tracing software tool was developed in order to simulate a specific environment of 11 buildings and take into account parameters, such as diffraction from building edges and rooftops. This approach concludes that the penetration loss is an increasing function of the elevation angle.