Rocky Mountain spotted fever has a mean incubation time of 7 days after the bite of an infecting tick.41 We need to remember that ticks can be very small; can attach on the body in places that are difficult to observe, such as the scalp, back, axillae, and inguinal regions; usually have a painless bite; and commonly go unnoticed. Diagnosis of RMSF can be difficult, particularly in the early stages of the illness. In a recent series of approximately 1000 cases of confirmed RMSF that occurred during 1993 to 1996, the triad of rash, fever, and headache was present in only 44% of the cases at any time during the illness. The occurrence of this classic triad at initial presentation, however, is less frequent, and the rash, which in the initial phases is macular rather than petechial, with the macules blanching with pressure, is generally not apparent until 3 to 4 days after the onset of the disease. Occasionally, RMSF may be "spotless" or "almost spotless."46 Of importance, early in the illness, more than 50% of the patients have nausea or vomiting, and infection of the gastrointestinal tract is a common misdiagnosis.23 Photophobia and myalgias, especially bilateral calf pain, can also be present.47,48 In addition, IgM and IgG antibodies reactive with R rickettsii may be undetectable during the first week of the illness.42 Tick-borne illnesses need to be considered by physicians during the evaluation of fever of unknown origin, especially in the spring and summer. However, cases have been reported in all 12 months. Other tick-borne diseases, including ehrlichioses, Lyme disease, and babesioses, can also pose diagnostic challenges to a clinician who is presented with a febrile patient with nonspecific symptoms such as headache, fever, and myalgias. Leukopenia, thrombocytopenia, or elevated liver enzyme levels may occur in patients with RMSF as well as other tick-borne diseases, such as the ehrlichioses.29,49- 52 A good history is essential to the diagnosis.