Between 1940 and 1976, two cases of tick-borne relapsing fever were reported in Colorado, but since 1977, 23 confirmed cases have occurred. All patients had fever, with a mean of 2.8 febrile episodes (range, one to six). Complications included thrombocytopenia, endophthalmitis, meningitis, abortion, in utero infection, and erythema multiforme. All treated patients were eventually cured with antibiotics, although two pregnant patients failed to be cured by their initial courses of antibiotics. Seven of 21 treated patients had Jarisch-Herxheimer reactions, three of whom required intensive care. Five of nine patients who received tetracycline at an initial dose of 5 mg/kg or more had reactions v none of four patients treated with lower doses. Possible causes of the recent increased incidence include increased physician awareness and reporting, improved diagnostic techniques, and an actual increase due to a larger population at risk. Because summertime visits to the Rocky Mountains are becoming increasingly popular, physicians elsewhere should know how to recognize and treat this condition.