We report an autopsy case of congenital monoblastic leukemia that developed in monozygotic twins. The twin presented with progressive hepatosplenomegaly at 4 weeks after birth. One twin died of massive bleeding and hypovolemic shock before the treatment started. At autopsy, the liver was diffusely enlarged and showed a diffuse whitish discoloration except for the subcapsular and perivenular areas. Microscopic examination disclosed infiltration of histiocyte-like atypical cells along the sinusoids and portal areas of the liver. Spleen, lymph nodes and choroid plexus were also infiltrated by the tumor cells. However, bone marrow involvement of the tumor was minimal although multifocal. On immunohistochemical staining, these atypical cells were reactive for CD68 (PGM-1) and lysozyme, suggesting that the tumor cells might have been derived from mono- histiocyte. Cytogenetic study revealed 9;11 translocation, which is frequently associated with acute monoblastic leukemia. To the best of our knowledge, this is the first report of congenital monoblastic leukemia of monozygotic twins in Korea.