The number of patients belonging to migrant populations who consult Lausanne's Hopital de l'Enfance (HEL) has increased massively in an exponential manner. HEL is a facility dedicated to children with a vocation of public health and training in a university setting and it has to make a point of developing an adequate and pertinent health care system for these populations. In order to do this, a study was undertaken in the form of a prospective survey including over a thousand ambulatory patients. Administrative data (origin, date of migration, nature of the permit and legal situation), social data (home language, professional situation, number of siblings), medical (diagnosis) and psychosomatic data (sleep, enuresis) was recorded as well as data testing the level of understanding between carrier and patient. The study allowed us to define priorities of intervention: introduction of a service of translators--cultural mediators trained by the association "Appartenances" (as well as on going follow-up and assessment of this service), training of health care workers in the fields of cultural mediation and the different aspects of migrant population medicine and, finally, the creation of a steering group within the Institution. Having allowed rapid and spectacular improvement in the quality of the service provided by the HEL, this process is also included in the will to improve health care given to migrant populations at local and national levels in accordance with the priorities defined by the WHO in this domain. It is this experience which is reported in this paper.