During epidemics of Dengue fever, Ro, the basic reproduction number of the disease, depends mainly on the contact rate between the vectors and susceptible people, and on the duration of the viremic phase of the illness. We will show that during real epidemics, the Ro may be misleading about the outcomes of a transmission model, especially when the proportion of « imported » vs. « autochthonous » cases is high, and mosquito control activities are efficient. A new method is presented to estimate the Ro from the initial phase of Dengue outbreaks and a model is proposed to take into account exogenous cases during epidemics. It is thus shown that epidemics may occur even if the Ro value is below one.