A 25 year old Chinese male presented with a three week history of fever (39.7ºC), viral upper respiratory symptoms, and numerous vesicular sores in the oral mucosa. Laboratory investigations revealed hemoglobin 11.9 g/dl, WBC 1.9x109/L, neutrophils 1.5x109/L, platelets 78x109/L, AST 274 U/L, ALT 528 U/L, LDH 4697 U/L, triglycerides 3.56 mmol/L, ferritin 33 784 µg/L. The blood film showed pancytopenia and many pelgeroid neutrophils (panel A) while the bone marrow showed florid hemophagocytosis (panel B). NK activity was reduced (0.1 lytic unit; normal >2.6). The bone marrow showed increased “double-negative” (CD3+/CD4-/CD8-; 23%, normal <10%) and NK (CD56+; 45%, normal <25%) cells, and normal male karyotype. Blood cultures, viral serology and viral swabs of the mouth sores were negative. A computed tomography scan of the abdomen revealed splenomegaly of 14.1 cm. He was diagnosed with hemophagocytic lymphohistiocytosis (HLH), and improved rapidly on etoposide-based chemotherapy. His neutrophil morphology also returned to normal. Adult HLH is rare but is being increasingly recognized. Morphologic dysplasia in blood and bone marrow has been reported, mostly in Asian patients, and is thought to be due to stem cell damage from hyperimmunity. Although distinguishing concomitant myelodysplastic syndrome may not be possible initially, particularly in older patients, these changes may improve after treatment of HLH.