A 77-year-old woman was admitted to our hospital complaining of high fever. The physical examination on admission indicated no abnormality. Although several antibiotics were administered, the spiking high fever was not alleviated. Two weeks after admission a macular rash appeared, and a high concentration of serum ferritin was observed. At this point, Still's disease was suspected, and the patient was referred to Nagasaki University Hospital. During the prescribed course of prednisolone, hepatic enzymes gradually increased to high titers accompanied by a positive test for cytomegalovirus (CMV) antigen. The CMV antigen disappeared after ganciclovir administration, and the concentration of serum ferritin decreased after steroid administration. In this report, we attempt to portray the relation between the pathogenesis of adult-onset Still's disease and the presence of CMV antigen.