Abstract Louse-borne relapsing fever seems to have become endemic in the southern Sudan. The epidemic history of the disease in the Sudan is reviewed. We have studied 363 Sudanese patients involved in an outbreak of louse-borne relapsing fever in Khartoum (Sudan) between January and June 1974. 318 of the 363 patients were new immigrants from the southern Sudan to Khartoum. The clinical presentation varied. The common clinical features of the disease were: fever (94%), headache (85%), hepatosplenomegaly (74%), body and joint pains (66%), abdominal pain and tenderness (63%), jaundice (46%) and epistaxis (40%). Thrombocytopenia was common. Biochemical evidence of hepatocellular and renal damage was present in most patients. The mortality rate was 5·5% with treatment. Post-mortem examination was performed on six cases. The organs predominantly involved were the liver, spleen, brain and lungs. The common causes of death were severe hepatic damage, lobar penumonia, subarachnoid haemorrhage and splenic rupture.