Since 1915 the front range of the Colorado Rocky Mountains has been postulated as a focus of endemic tick-borne relapsing fever. However, the disease has rarely been identified: only two cases have been reported in Colorado since 1944. Three sporadic cases in 1977--tightly grouped geographically and temporally--prompted an epidemiologic review. Tick-borne relapsing fever should be considered in the differential diagnosis of recurrent paroxysmal fever--with or without known presence of ticks--whenever exposure in an endemic area is part of a patient's history.