This prospective study to assess the antibody titers in 102 patients with scrub typhus showed that the antibody positivity rates within 5 days of symptom onset were 44.1, 35.3, and 57.6% when cutoff values were used ≥1:16 for IgM, ≥1:256 for IgG, and ≥1:40 for total Ig, respectively. The gene discussed is CD40LG; the disease is scrub typhus.