A measuring campaign for the assessment of electromagnetic exposure levels from mobile phones in the city center of Leuven, Belgium, has been carried out. The main objective of the assessment is to study the dependency of the exposure of the user by his own mobile phone in terms of location in the city (very close to base stations and at randomly selected locations). The measurements were performed in both public and private areas in 60 outdoor and 60 indoor locations in Leuven. The campaign was focused on GSM 900 mobile communications. The results show that the exposure is considerably higher for indoor environments compared to outdoor environments, and at the randomly chosen locations compared to locations very close to base stations. However, the most important observation is that the average outdoor exposure in Leuven of the user of a mobile phone is about 8 times higher than the average outdoor exposure by base stations. Indoors, this factor rises to about 30.