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Colorado tick fever (CTF) is a biphasic, febrile illness caused by a Coltivirus and transmitted by the Rocky Mountain wood tick, Dermacentor andersoni, in the western United States and Canada. Symptoms generally include acute onset of fever, headache, chills, and myalgias; illness often lasts for 3 weeks or more. Laboratory-confirmed cases of CTF were identified from public health department records in Montana, Utah, and Wyoming, and from the Centers for Disease Control and Prevention diagnostic laboratory records. Additional descriptive epidemiologic data were obtained by medical record abstraction. Ninety-one cases were identified from 1995 to 2003, resulting in an overall annual incidence of 2.7 per 1,000,000 population. The annual incidence decreased over the 9-year study period. Cases were 2.5 times more frequent in males than females. The highest incidence of cases occurred in persons aged 51-70. Tick exposure prior to illness onset was reported in 90% of the cases in which a more detailed history was available. The most common symptoms were fever, headache, and myalgia; 18% of the case patients were hospitalized. While there has been an overall decline in the recognized incidence of CTF cases, the reasons for the decline are unknown. Possibilities include a reduced intensity of surveillance and a true decrease in incidence. As more people continue to visit, move to and work in endemic areas, CTF should be considered in anyone presenting with a febrile illness following tick exposure in an endemic area. Heightened awareness for the disease and tick prevention messages should be part of public health measures to further decrease the incidence of disease.