The colonization of the forested areas intersected by the Trans-Amazon highway was started in mid-1971. The immigrants originated from different areas of the country [Brazil], but most of them came from the northeast. The colonization was intense in Altamira, Maraba and Itaituba, all in Para State. By the end of 1974 about 28,000 colonists and their families were living in these areas. It was assumed that these people would come into contact with certain zoonotic agents such as arboviruses and leishmanias, and that agents and vectors of infectious diseases previously unknown in the Trans-Amazon area, such as schistosomiasis and Chagas'' disease, might be introduced. Investigations commenced in 1972 to identify certain health problems in different areas of the highway. In Nov. 1972 a permanent surveillance program was initiated in the area of Altamira. Serological studies demonstrated that arbovirus infections occurred among colonists. Of the persons studied, mainly in hospitals and health posts, 2.6% were infected in the 1st semester of 1973, 6.5% in the 2nd semester of 1973 and 18.0% in the 1st semester of 1974. Mayaro, Guaroa, Itaporanga and Oropouche were the commonest arboviruses responsible for the infections. A thrombocytopenic purpura, named hemorrhagic syndrome of Altamira (HSA), was noticed in the Altamira area from the beginning of 1972. Most cases were restricted to the colonists and about 160 cases were observed up to Oct. 1974; the disease was responsible for 3 deaths. Epidemiological and serological evidence suggests that the syndrome is associated with the black-fly Simulium and its bite. Studies on leishmaniasis revealed a high incidence of infections among wild animals trapped at various sites along the Trans-Amazon highway. Of 166 specimens of the rodent Proechimys studied, 26 were infected with Leishmania mexicana amazonensis (15.7%), and the L. braziliensis complex was isolated from Proechimys, and the opossum Didelphys. Skin tests on colonists along the highway show a high incidence of positive reactions to leishmanin (up to 60%), but active cases of cutaneous leishmaniasis are uncommon. The incidence of the disease may rise sharply in the succeeding, nonimmune generation. Dense populations of highly anthropophilic sandfly species were seen at all points studied along the new roads. Some of these species will undoubtedly act as vectors. Between 1971-1974, 310 imported cases of schistosomiasis were diagnosed in Altamira and Maraba and all were treated with etrenol. Although 3 foci of planorbid snails were discovered, there is no evidence yet of local transmission of the disease. Malaria is a very important problem along the Trans-Amazon highway and thousands of cases were diagnosed. Most infections are caused by Plasmodium falciparum but infections with P. vivax are also common. Leptospira antibodies were detected among persons living along the road and in wild animals. Cases of South American blastomycosis were detected in the area between Maraba and Altamira, and Histoplasma was isolated from several wild animals.