BACKGROUND: Extramedullary tumors of lymphoid and myeloid blasts outside the well-defined sanctuaries following allogeneic bone marrow transplantation (allo-BMT) are rare. Little is known about the biology, treatment, and outcome of these tumors in this setting. METHODS: In this retrospective analysis, 134 consecutive patients with acute myeloid leukemia (AML) or chronic myeloid leukemia (CML) who underwent allo-BMT at a single institution between 1990 and 1998 were reviewed. Five cases of isolated extramedullary myeloid sarcoma that occurred as patterns of recurrence following allo-BMT between 1990 and 1998 are reported. These patients were treated with radiotherapy, systemic chemotherapy, or a second allo-BMT. Clinical outcome is compared with posttransplantation bone marrow relapses observed during the same period at the same institution. The literature on the clinical characteristics, currently available treatment, and outcome of posttransplantation myeloid sarcoma patients was reviewed. RESULTS: Excluding isolated skin and central nervous system recurrences, the frequency of extramedullary myeloid sarcoma encountered as a relapse pattern following allo-BMT was determined to be 3.7% among patients with acute or chronic leukemia of myeloid origin. The survival of patients who were managed with radiotherapy and systemic chemotherapy was less than 4 months. A patient who underwent a second allo-BMT following local radiotherapy is alive and in complete remission more than 33 months after the diagnosis of myeloid sarcoma. The median survival of 17 patients with posttransplantation bone marrow relapse following allo-BMT was 2.2 months. When posttransplantation medullary recurrences are analyzed, patients with CML had a median survival of 12 months, with a significantly better 5-year survival rate than patients with AML (0 vs. 60%, P = 0.015; median survival, 12 months). CONCLUSIONS: The clinical outcomes of patients with recurrent isolated extramedullary myeloid sarcoma following allo-BMT are poor, as in any leukemic relapse, with the exception of patients with CML in this setting.