To access this article, click on "Additional Links"BACKGROUND: Although endemic in parts of southern Sudan, visceral leishmaniasis (VL) had not been reported in Western Upper Nile (WUN) until an epidemic was confirmed in 1989. A combination of circumstances created conditions for transmission among a population of mainly Nuer and Dinka people who had no immunity. The civil war which restarted in 1983 has been a major contributing cause and continues to hinder provision of treatment, data collection and control measures. METHODS: Since the first of three clinics to treat VL was established in WUN in 1989, data on the epidemic and mortality have been collected in seven retrospective surveys of villages and among patients. Adults were interviewed about surviving family members and those who had died since the epidemic came. Survey death rates are used here to estimate mortality from VL and 'excess mortality' above expected levels. RESULTS: The surveys found high mortality at all ages and suggest an overall death rate of 38-57% since the epidemic started in 1984, and up to 70% in the most affected areas. Both methods of estimation suggest that around 100,000 deaths, among about 280,000 people in the epidemic area, might be attributable to VL. CONCLUSIONS: This continuing epidemic has shown that VL can cause high mortality in an outbreak with astonishingly high infection rates. Population movement has been a major factor in transmission and poor nutritional status has probably contributed to the risk of clinical infection. Although over 17,000 people have been successfully treated for VL at the clinics in WUN, the disease is likely to become endemic there.