Epidemiology of dengue fever is changing dramatically. The worldwide incidence is rising and clinical symptoms are worsening. Reports describing forms associated with haemorrhages or shock syndrome involving both children and adults are increasingly frequent in regions beyond Southeast Asia where the first cases were observed. Many mechanisms could be implicated in these changes, including modifications of the virus, host, vector, or socio-economic factors. The new facilities in the laboratory diagnostic (MAC-ELISA; molecular biology), the commercialization of these assays allow not only assessment of morbidity and mortality in endemic areas and early detection of epidemic outbreaks but also evaluation of socio-economic impact and effectiveness of control measures. Now, the efficiency of the fight must be better, otherwise dengue will grow up during this new century.