The flight of 500,000-800,000 Rwandan refugees into the North Kivu region of Zaire in July, 1994, overwhelmed the world's response capacity. During the first month after the influx, almost 50,000 refugees died, an average crude mortality rate of 20-35 per 10,000 per day. This death rate was associated with explosive epidemics of diarrhoeal disease caused by Vibrio cholerae 01 and Shigella dysenteriae type 1. 3-4 weeks after the influx of refugees, acute malnutrition rates among children under 5 years old ranged between 18 and 23%. Children with a recent history of dysentery and those in households headed by women were at higher risk of malnutrition. A well-coordinated relief programme, based on rapidly acquired health data and effective interventions, was associated with a steep decline in death rates to 5 to 8 per 10,000 per day by the second month of the crisis. The prevention of high mortality due to diarrhoeal disease epidemics in displaced populations relies primarily on the prompt provision of adequate quantities of disinfected water, basic sanitation, community outreach, and effective case management of ill patients. In the emergency phase, effective, low-technology measures include bucket chlorination at untreated water sources, designated defaecation areas, active case-finding through community outreach, and oral rehydration. Relief agencies must place increased emphasis on training personnel in relevant skills to address major public health emergencies caused by population displacement.