Although Information and Communication Technologies (ICT) are presented today as a chance for the environment, further research is needed to understand the extent of their impacts. Essentially, little research has been done on the environmental impacts of ICT infrastructures. Their assessment thus still faces some methodological problems which are dealt with in this paper, notably in the case of Wireless Sensor Networks (WSN). We provide a method to analyse WSN in an environmental perspective, which clarifies two important issues: the definition of the system lifecycle and the synergic behaviour of its elements. This method is applied to a concrete case, results are generated and eco-design alternatives are presented and evaluated. The results show the importance of adopting a system-level perspective while performing environmental assessment of ICT. It helps determine the most influential system parameters, bringing broader understanding and eco-design perspectives.