Dengue is a viral disease with clinical features that vary in intensity according to certain host and viral strain characteristics. Dengue hemorrhagic fever is practically a new clinical entity for the majority of physicians in the Americas and therefore the need to disseminate the most important clinical and epidemiological features that allow for the opportune diagnosis and therapy. The clinical spectrum of the disease includes asymptomatic to severe hemorrhagic forms and each stage has its own signs and symptoms. This clinical description explores the physiological mechanisms involved in the pathogenesis of dengue hemorrhagic fever. This paper gives special emphasis to the alarm signs that foretell the presentation of the shock syndrome requiring emergency treatment and advances a classification scheme for epidemics. This scheme identifies the type of treatment and which patients need observation, laboratory tests and hospital care.