Aims. To describe AIDS and malaria geography in Brazil, highlighting the role of injecting drug users (IDUs) in malaria outbreaks occurring in malaria-free regions, and the potential clinical and public health implications of malaria/HIV co-infection. Design. Review of the available literature and original analyses using geoprocessing and spatial analysis techniques. Findings. Both HIV/AIDS and malaria distribution are currently undergoing profound changes in Brazil, with mutual expansion to intersecting geographical regions and social networks. Very recent reports describe the first clinical case of AIDS in a remote Amazonian ethnic group, as well as malaria cases in Rio de Janeiro state (hitherto a malaria-free area for 20 years); in addition, two outbreaks of both infections occurred at the beginning of the 1990s in the most industrialized Brazilian state (Sao Paulo), due to the sharing of needles and syringes by drug users. Spatial data point to: (a) the expansion of HIV/AIDS towards malarigenic areas located in the centre-west and north of Brazil, along the main cocaine trafficking routes, with IDU networks apparently playing a core role; and (b) the possibility of new outbreaks of secondary malaria in urban settings where HIV/AIDS is still expanding, through the sharing of needles and syringes. Conclusions. New outbreaks of cases of HIV and malaria are likely to occur among Brazilian IDUs, and might conceivably contribute to the development of treatment-resistant strains of malaria in this population. Health professionals should be alert to this possibility, which could also eventually occur in IDU networks in developed countries.