The age profiles of the infected populations of two dengue hemorrhagic fever (DHF) epidemics, the 1997 epidemic, in Santiago de Cuba and the 1998 epidemic in Thailand, are compared. Using an age-structured model of disease transmission, the dependence of the forces of infection on age was determined for each epidemic. The difference in the behavior of the two epidemics and the role of primary and secondary infection in the development of DHF are discussed.