Relapsing fevers occur worldwide and are characterized by recurrent episodes of fever and spirochetemia. In central, eastern, and southern Africa, the disease is often caused by Borrelia duttonii, which is transmitted by the soft tick Ornithodors moubata. We conducted a field investigation in September 1994 at a hospital in Mitwaba, southern Zaire, which was the only medical facility within 150 km. The introduction of a rapid blood-smear staining technique allowed us to demonstrate that 4.3%-7.4% of the 25-50 new outpatients seen each day had relapsing fever. Because of the absence of malaria in this area, these patients account for most of the febrile patients. The incidence of relapsing fever among all pregnant women in the maternity ward was estimated to be 6.4%, and this condition often led to maternal death or to spontaneous abortion. The 16S rRNA gene of B. dutonii was sequenced after the spirochete was isolated from patients' blood samples and directly from Ornithodoros tick vectors. In this region of Africa, relapsing fever should now be considered an important public health priority.