Reactive hemophagocytic syndrome (HPS) is characterized by hemophagocytosis by activated histiocytes, resulting in pancytopenia and liver dysfunction. We describe a patient with adult onset Still's disease (AOSD) in whom HPS developed. An 80-year-old Japanese woman with high fever, arthralgia, skin rash, and pleuritis was admitted to our hospital for further examination. She was diagnosed with AOSD and steroid therapy was initiated. During the course of steroid therapy, a re-elevation of serum ferritin levels and a marked increase in serum transaminase were observed. Bone marrow aspiration revealed an increase in the number of histiocytes with hemophagocytosis and cytomegalovirus (CMV)-positive leukocytes were detected. At this time we diagnosed the patient as having virus-associated hemophagocytic syndrome (VAHS) and elevated levels of trasaminase and ferritin were normalized by ganciclovir treatment. Reactive HPS occurs in cases of active AOSD. However, it should be noted that HPS may be accompanied by opportunistic infections during immunosuppressive therapy requiring prompt antibiotic therapy.