Granulocytic sarcoma is considered to be rare and its frequent occurrence is associated with specific genetic changes such as t(8;21). To investigate an association between MLL (mixed lineage leukemia or myeloid-lymphoid leukemia) rearrangement and granulocytic sarcoma, we applied fluorescence in situ hybridization for detection of the 11q23/MLL rearrangements on the bone marrow cells of 40 patients with childhood acute myelogenous leukemia (AML). Nine (22.5%) of 40 patients exhibited MLL rearrangements. Three (33.3%) of these nine patients had granulocytic sarcoma and were younger than 12 months of age. Of these three patients one presented as granulocytic sarcoma of both testes with cerebrospinal fluid involvement, the second case presented in the form of an abdominal mass, and the third as a periorbital granulocytic sarcoma. On the other hand, no granulocytic sarcomas were found among MLL-negative patients. It is likely that MLL-positive infant AML may predispose granulocytic sarcoma. Regarding the findings of our study and those of other reports, we would guess that the incidence of granulocytic sarcoma in pediatric MLL-positive AML may be equal to or greater than the 18 to 24% described in AML with t(8;21). Further investigations designed to identify 11q23/MLL abnormalities of leukemic cells or extramedullary tumor may be helpful for the precise diagnosis of granulocytic sarcoma.