A 2-year-old boy developed proptosis of the right eye in May 1982. He was admitted to Isawa Hospital on June 9. Physical examination revealed a well developed child with slight proptosis of the right eye and swelling of the right temple. Routine serologic and blood examination revealed no abnormality except LDH, ALP and LAP. CT scan showed high density area in the anterior part of the right temporal lobe and lateral wall of the right orbit. He underwent open-biopsy of this tumor on July 15. Eosinophilic granuloma was suspected, but definite diagnosis was not made. Second biopsy was performed at right upper gingiva on August 2. Follow-up CT scan revealed tumor invaded sphenoidal and maxillary sinus. The patient was transferred to Tohoku University Hospital on August 13. On admission myeloblasts appeared in peripheral blood, and bone marrow examination revealed acute myelo-monocytic leukemia. We discussed its difficulty of differential diagnosis in literature.