Adult Still’s disease is an important differential diagnosis of pyretic disease and it does not necessarily appear to be a distinct disease entity. The etiology of adult Still’s disease is not yet known. However, it has been considered that adult Still’s disease may be triggered by certain infections, such as the Coxsackie, parvo B19, rubella, mumps, Epstein-Barr, and cytomegalo virus, as well as mycoplasma, toxoplasma, and so on. Recently, we experienced a patient with adult Still’s disease with an increased Chlamydia pneumoniae antibody titer. The titer decreased slowly after the beginning of steroid therapy, associated with improvement of clinical symptoms. In this report we mention the relationship between the pathogenesis of adult Still’s disease and a high titer of Chlamydia pneumoniae antibody.