The mainstay of diagnosing scrub typhus is through serologic testing. However, because of the delay of several days from the onset of the illness to the increase in the antibody titer, diagnosis can be difficult in the early stage of the illness. The aim of this study was to determine the clinical usefulness of adenosine deaminase activity (ADA) in diagnosing scrub typhus. A total of 104 patients were enrolled during the study period. Of these patients, 59 patients were diagnosed with scrub typhus and the mean serum ADA level was 46.7±11.6 U/L. High serum ADA levels strongly supported the diagnosis of scrub typhus, especially in patient without eschar. The remaining 44 patients did not have scrub typhus and 95% (42/44) of them had normal serum ADA levels. Therefore, evaluating serum ADA level could help diagnose patients with acute febrile disease such as scrub typhus, especially in its early stage of disease progression.