Telework is often suggested as an instrument to improve sustainability by reducing environmental and socio-economical impacts of mobility on society. Currently, telework is however not yet implemented as a widespread measure in companies in Belgium. Goal of this paper is to determine if further encouragement of telework is indeed desirable from a sustainable mobility viewpoint and whether it should be supported by future policies. Based on survey data, an appraisal of the environmental, mobility and socio-economic impacts of telework for companies located in the Brussels Capital Region (BCR) is performed. Traffic on the road network in and around the BCR is heavily congested during peak periods and every additional vehicle causes additional externalities. Congestion, climate change, air pollution, noise, traffic accidents and externalities linked to up-and downstream processes are the most well known transport related externalities, and are taken into account in the calculations. Survey data was generated through a questionnaire that was distributed to both workers and management of six large companies, whose main offices are located in the BCR and where teleworking is already practiced. For these companies, the external costs of trips to the central office are compared to the external costs of trips to satellite offices and the external costs caused by additional distances travelled when teleworking at home. Modal shifts occurring between trips travelled to the central office and trips travelled to the satellite office are taken into account and play an important role in the overall impact on external transport costs. Also receptor density and congestion levels along the routes travelled are taken into account. Results are calculated for the different modal choice scenarios, as well as on an aggregated level.