Background：Tsutsugamushi disease, is an acute febrile illness caused by the infection of Orientia(O.) tsutsugamushi which is transmitted by mites such as Leptotrombidium(L.) pallidum. Because the antibody titer against O. tsutsugamushi has not risen enough to be detected at the initial stage of tsutsugamushi disease, the early serologic diagnosis of this disease has some difficulties. Objective：The aim of this study was to show that the recently introduced technique of polymerase chain reaction(PCR) is excellent for the detection of rickettsial DNA in the patient's blood at the acute phase of illness. Methods：Nested PCR with Boryong specific 56kDa protein antigen was done with sera from twenty one patients with tsutsugamushi disease and with two groups of mites(L. pallidum). The presence of O. tsutsugamushi DNA in the peripheral blood mononuclear cells of patients with tsutsugamushi disease was also determined by nested PCR during the antibiotic treatment. Results：1.Positive results with nested PCR with Boryong specific 56kDa protein antigen were found in eleven out of twenty one patients with tsutsugamushi disease and two samples of L. pallidum extracts. 2.r the initiation of antibiotic treatment, all samples(11 patients) were positive on the second day, and seven samples were positive on the seventh day, showing a slow action of the drugs against the rickettsia within the cells. Conclusion：Nested PCR is an ideal method for the detection of O. tsutsugamushi DNA during the early phase of tsutsugamushi disease. (Korean J Dermatol 2001;39(12)：1383~1390)