Behaviours such as diet, sedentariness and how much exercise we do play a major role in influencing health and wellbeing. As a medium, mobile serious games have the potential to educate and foster behaviour change whilst engaging and motivating users. This paper illustrates the user-centred design approach used to develop a mobile serious game to promote healthy lifestyles. In particular, this research, developed in the frame of the PEGASO European project, focuses on teenagers and healthy behaviours linked to the prevention of obesity. In this approach, teenagers from different European countries (Italy, Spain, and United Kingdom) participate to the co-design of the serious game. This activity is still ongoing but first findings show encouraging feedback concerning the generic game mechanics and the designed game.