A total of 520 European travelers with suspected dengue fever were examined, and acute dengue virus infection was confirmed in 127 of them. Molecular and serological tests for dengue diagnosis, according to their usefulness in the different stages of the disease, were performed. The accuracy of the IgM/IgG ratio and the IgG avidity index was confirmed during the early phase of the illness to discriminate the serologic status (primary versus secondary immune response) of patients who were either naive or previously vaccinated against other flaviviruses. Virologic markers of secondary infection were detected in 11.8% of nonvaccinated infected patients and in 92.6% of yellow fever vaccinated patients. The proper use of these simple methodologies could help in the identification of patients affected with secondary infection, who may be at a higher risk of developing a hemorrhagic disorder. © 2009 Elsevier Inc. All rights reserved.