BACKGROUND: The present study had three aims: to estimate for the first time with standardized methodology allowing international comparisons the prevalence and the severity of childhood allergies in Reunion Island, to determine their risk factors and to make a comparison with metropolitan France. METHODS: The same standardized protocol (ISAAC) was used in population-based samples of adolescents recruited in metropolitan France between 1993 and 1994 (n=18555) and in Reunion Island in 2000 (n=2 362). RESULTS: Compared to metropolitans, the adolescents living in Reunion Island suffered significantly more from asthma in the past Year and in life (22 vs 13% and 19 vs 13% respectively). The trend was less clear for allergic rhinitis and eczema since in the case of these manifestations the adolescents reported more symptoms (27 vs 16% et 14 vs 10%, respectively) whereas the metropolitans reported more lifetime diagnostic labels (14 vs 17% et 18 vs 23%, respectively). In Reunion Island, allergies were slightly more severe (1.1 vs 0.8% for severe asthma and 15 vs 2% for eczema). Furthermore, 68% of asthmatics who had severe asthma attacks in the past Year and more than a third of adolescents with severe asthma did not take any asthma medication, which is different from the metropolitan situation. Risk factors for allergies were the same in Reunion Island and in metropolitan France, except the ownership of a pet in the first Year of life, which was not protective against the development of allergies as recently shown in some western countries but was a risk factor. CONCLUSIONS: Asthma and allied diseases are less frequent in metropolitan France than in Reunion Island where management appears to be insufficient. Improvement in prevention and management plans are required in Reunion Island.