An outbreak of coccidioidomycosis occurred in a US Marine reserve unit based in Tennessee after a 3-week training exercise in California that involved substantial exposure to soil and dust. Interviews and serologic testing were done on three occasions (6, 11, and 15 weeks) after the men returned from California, and spherulin skin tests were done at 6 months. Of 27 men, 8 (30%) had evidence of recent coccidioidal infection. Of these, 7 (88%) had an illness consistent with coccidioidomycosis that, despite medical evaluation, was diagnosed incorrectly in 5 men (71%). Diagnosis of coccidioidal pneumonia outside an area in which Coccidioides immitis is endemic is unlikely unless the health care provider is aware that the patient traveled recently. Detection of coccidioidomycosis could be facilitated if organizations that regularly send people to C. immitis-endemic regions were to inform these persons about the risks of infection.