Background: Hemophagocytic syndrome (HS) is a distinct clinical entity characterized by high fever and hemophagocytosis with histiocytosis in tissue biopsy. We seldom encounter patients who suffer from unexplained, persistent fevers. Although there have been many studies about childhood HS, studies about adult HS are relatively rare. The causes and prognoses of HS in adults were evaluated in this study. We focused on infection-related HS. Material and Methods: We enrolled 41 adult patients with HS retrospectively from four hospitals in Kyungbuk province and Daegu city. The patients were diagnosed by bone marrow or liver biopsy, either of which showed hemophagocytosis with histiocytosis and had clinical findings consistent with HS. We explored the etiologies, clinical symptoms, laboratory findings, treatments, and outcomes of each case. Results: The most common cause of HS was infection, such as the Epstein-Barr virus (EBV) or Mycobacterium tuberculosis. Old age and malignancy-associated HS had a poor prognosis. The overall mortality rate was 17.1%. Most patients survived after conservative therapy and the control of underlying diseases, in contrast to previous studies that showed a poor prognosis of infection-associated HS. Conclusions: A proper investigation is crucial to determine the cause of HS in patients who have unexplained persistent fever and hemophagocytosis with histiocytosis in their tissue. Cases of infection-related HS are common, but physicians should consider undiagnosed malignancy that may be related to a poor prognosis. Treatments appropriate to the causes are important for better outcomes in adult HS.