In 2005, a 12-week health promotion campaign in Québec utilized a multimedia communications strategy in which a website was used to supply information and to serve as an accompaniment to support individual engagement with the campaign. The "Défi Santé 5/30" or "5/30 Health Challenge" was intended to reinforce behaviours related to healthy eating and the regular practice of physical activity. This communications strategy offered the public the ability to personally engage in a "health challenge" by fixing objectives to attain over the course of the campaign. The campaign's media use and impact are evaluated with the aid of a telephone survey (n = 609), discussion groups with individuals (n = 102) who had been exposed to the campaign, and discussion groups with health professionals (n = 32). The article explores underlying explanations for behavioural changes during the campaign with a focus on the role of the website. Several characteristics can potentially contribute to the development of motivation and fidelity in users: specific objectives of the website; content; architecture and usage techniques; media convergence and contribution of partners; an accompaniment approach that is at once virtual, media-based, interpersonal while also using contributions from health professionals. Non-facilitating factors are also discussed: technical and socio-cultural accessibility; ethics and constraints related to the participation of health professionals. The "Défi Santé 5/30" has demonstrated that a website utilizing virtual, media-based, interpersonal and technical accompaniment can contribute to behavioural change.