Visceral leishmaniasis (VL) was known to be endemic in Somalia along the basins of the (Middle) Shebelle and (Lower) Juba rivers, and in Kenya in parts of the Rift Valley, on the border with Uganda and the Eastern Provinces. From May 2000 to August 2001, we diagnosed 904 patients with VL. The patients came from an area which spanned the Wajir and Mandera districts of north-eastern Kenya, southern Somalia, and south-eastern Ethiopia. Small numbers of patients were also seen in northern Somalia. These areas were either previously non-endemic for VL, or had only sporadic cases prior to the epidemic. We describe the features of the outbreak and review the history of VL in the region. Unusual rainfall patterns, malnutrition, and migration of a Leishmania-infected population seeking food and security may have contributed to this outbreak.