Three cases of tick-borne fever diagnosed during the summer of 1979 are reported and the ecoepidemiology, clinical manifestations, and treatment of this infection are reviewed. Although challenging, the diagnosis can be made easily if specific historical clues are sought and the patient's blood smear is carefully examined. The diagnosis of this condition early in its course can save clinicians and patients the anxiety and cost of the work-up of a "fever of unknown origin." Since vacationing in the national parks and forests has become increasingly popular among many American families, tick-borne relapsing fever should be considered in any patient with an acute or recurrent fever of unknown origin who exhibits nonspecific symptoms of an undifferentiated "viral illness," and who gives a history of sleeping overnight in log cabins in the coniferous forests of the Western mountains of the United States.