The emergence of dengue haemorrhagic fever is a public health problem in Latin America and the Caribbean. This study, carried out in French Guiana where malaria is endemic, evaluated the value and the limitations of a non-specific alert system including all patients admitted to the emergency department of Cayenne Hospital, between 1 January 1996 and June 2001. Four indices were studied on a weekly basis: the emergency malaria negative index (EMN), the EMN thrombocytopenia index (EMNT), the dengue suspected index: EMNT/EMN ratio; and the number of hospitalized patients with dengue fever according to the Department of Medical Information. These indices were retrospectively compared with data from the Arbovirus Reference Centre at the Pasteur Institute in French Guiana. Using the non-specific indices, we were able to identify four clear epidemics, two of which were shown to be linked to dengue. Variations in the incidence of malaria had no marked effect on this alert system. We propose that this simple, cheap, sensitive and reactive alert system be used to improve the serological and virological monitoring of dengue and to facilitate adequate and timely vector control measures. It could be used in all regions at risk of dengue and malaria.