In February 1998, an outbreak of acute febrile illness was reported from the Kapalata military camp in Kisangani, the Democratic Republic of Congo. The illness was characterized by an acute onset of fever associated with severe headache, arthralgia, backache, neurologic signs, abdominal pain, and coughing. In 1 individual, hemorrhagic manifestations were observed. The neurologic signs included an altered level of consciousness, convulsions, and coma. Malaria was initially suspected, but the patients showed negative blood films and failed to respond to antimicrobial drugs. A total of 35 sera collected from the military patients in the acute phase were tested for the presence of IgM against vector-borne agents. Serum IgM antibodies against West Nile fever virus were found in 23 patients (66%), against Chikungunya virus in 12 patients (34%), against dengue virus in 1 patient (3%), and against Rickettsia typhi in 1 patient (3%). All sera were negative for IgM antibody against Rift Valley fever virus, Crimean Congo hemorrhagic fever virus, and Sindbis virus. These data suggest that infections with West Nile fever virus have been the main cause of the outbreak.