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Web designers have to create websites that fit the users? needs as well as the client?s needs, which may be sometimes conflicted. Taking into account these two viewpoints (users and client) reflects the management of various constraints, such as aesthetic constraints or ergonomic ones. More precisely, concerning the users? needs, recent studies showed that web designers experienced difficulties in taking into account the needs of the future users while designing. In order to support their activity, a knowledge-based system was created and provided to designers. This system was composed of ergonomic knowledge to help designers creating and evaluating their web pages. The present study aims at determining the influence of this kind of tool on the cognitive activity of professional web designers, on the ergonomic quality of their productions and on the aesthetics of these productions. The results show that the web designers did not very often use MetroWeb while designing web pages, however, rather surprisingly, the designers who did use MetroWeb more often exhibited a user-centered creativity activity than those working without MetroWeb. Moreover, the web pages created by designers with MetroWeb were evaluated as more aesthetic than the web pages created by designers without MetroWeb. We conclude this paper by discussing these findings, and suggesting future possible ways of research intended to assist designers to adopt a user-centered approach to their activity without negliting aesthetics and creativity.