A 14-year-old female exhibited an acute vulvar ulcer during the course of hemophagocytic syndrome (HPS). The patient presented persistent high fever and a deep painful vulvar ulcer lasting for more than 2 weeks. Neither infection with sexually transmitted agents nor autoimmune disorder were found to be positive. The presence of hemophagocytosis in the bone marrow and elevation of urinary beta-2-microglobulin (beta-2M) prompted the diagnosis of HPS. Acute vulvar ulcer is rare, but it should be recognized as a mucous manifestation of HPS. During the clinical course, urinary beta-2M was the most sensitive marker for the evaluation of the disease activity of HPS.