From September through December 2005, an outbreak of hemorrhagic fever occurred in South Kordofan, Sudan. Initial laboratory test results identified IgM antibodies against yellow fever (YF) virus in patient samples, and a YF outbreak was declared on 14 November. To control the outbreak, a YF mass vaccination campaign was conducted and vector control implemented in parts of South Kordofan. Surveillance data were obtained from the Sudan Federal Ministry of Health. Clinical information and serum samples were obtained from a subset of patients with illness during the outbreak. Nomads, health personnel and village chiefs were interviewed about the outbreak. Mosquitoes were collected in 11 villages and towns in North and South Kordofan. From 10 September to 9 December 2005 a total of 605 cases of outbreak-related illness were reported, of which 45% were in nomads. Twenty-nine percent of 177 patients seen at clinics in Julud and Abu Jubaiyah had illness consistent with YF. Five of 18 unvaccinated persons with recent illness and 4 of 16 unvaccinated asymptomatic persons had IgM antibodies to YF virus. IgM antibodies to chikungunya virus were detected in five (27%) ill persons and three (19%) asymptomatic persons. These results indicate that both chikungunya and YF occurred during the outbreak.