Bartonella spp are fastidious bacteria that occur in the blood of man and mammals; they are usually vector borne but can also be transmitted by animal scratches and bites. The bartonelloses of medical importance comprise Carrión's disease, trench fever, cat-scratch disease, bacillary angiomatosis, and peliosis hepatis. Carrión's disease, known as Oroya fever in the acute phase and verruga peruana (Peruvian wart) in its chronic form, has curious manifestations that, until recently, have been restricted in their geographic distribution to dwellers of the high, dry Andean valleys, but new sites of disease are emerging. Trench fever is associated with louse-borne disease and homelessness. Cat-scratch disease, bacillary angiomatosis, and peliosis hepatis are increasingly being recognized as causes of human disease, especially in susceptible population groups such as HIV-infected persons. The Bartonella spp are considered emerging human pathogens. The clinical manifestations, differential diagnosis, laboratory diagnosis, and treatment of these conditions are discussed.