As the AIDS pandemic spreads to rural areas and human visceral leishmaniasis (VL) becomes more common in suburban areas, there is an ever greater degree of overlap between the geographical distributions of the two diseases and, in consequence, an increasing incidence of Leishmania/HIV co-infection. Cases of the co-infection have been reported from 35 countries around the world but most have been recorded in south-western Europe. There has been a total of 1911 cases detected in Spain, France, Italy and Portugal. The incidence of Leishmania/HIV co-infection is expected to continue increasing in eastern Africa but to fall in south-western Europe as increasing numbers of HIV-positives in the latter region are given the new, highly active, antiretroviral therapy (HAART). In 1998, a world-wide network of surveillance for the co-infection, which now includes 28 member institutions, was established by the World Health Organization (WHO) and the Joint United Nations Programme on HIV/AIDS (UNAIDS). In south-western Europe, the surveillance system is based on 16 institutions and is already well established. The systematic use of standardized and recently computerized case-report forms, a central international registry at the WHO's headquarters in Geneva, and the use of a geographical information system (GIS) for mapping and monitoring the co-infections have together improved the overall quality of the epidemiological data-gathering. All member institutions of the global network report to the WHO on an annual basis. The data collected are then analysed and periodically disseminated through international publications. The GIS allows the relevant epidemiological and demographic data-sets to be integrated and permits all detected cases of co-infection to be mapped down to locality level. The system also allows the spatial distribution of cases to be visualised and analysed and the geographical spread of the co-infection to be monitored over time. The risk posed by co-infected patients, as a source of Leishmania infection for the sandflies feeding on them, has recently been confirmed. The parasites and HIV may also be transmitted as the result of needle-sharing among intravenous-drug users.