The aim of this study was to describe the presenting features, the frequency and outcome of myeloid sarcoma (MS) diagnosed in our Institution from January 1995 to December 2000. Twelve MS were seen and the frequency account for only 2% of all acute myeloid leukemia (AML) patients observed in our department in the same period. Median age was 45 years (range: 4-84). All had been initially misdiagnosed as malignant lymphoma (ML) and a median of 2.9 months (range: 1-6) elapsed between the misdiagnosis and the correct of MS, effectuated in our department. At that time, a bone marrow examination revealed a myelodysplastic condition in seven patients, an infiltration by blast cells >30% in two patients, and normal features in the other three. In the non-leukemic patients a median of 5 months (range: 2-44 months) elapsed between the diagnosis of MS and acute leukemia. In all, 10 patients received intensive treatment. A total of seven patients (70%) achieved MS complete remission (CR). Patients who presented isolated skin localization and received only radiotherapy, obtained a MS-CR, but subsequently developed AML. Only in patients who were treated within 4 months from the initial ML diagnosis we achieved complete remission of both MS and leukemia, whereas in patients who were treated after this time, we obtained a complete disappearance of MS without response at the bone-marrow level, irrespectively of the specific therapy regimen. Median survival time from MS diagnosis was 7 months (range: 1-49 months), and only one patient is still alive, 49 months after bone marrow transplantation. Our data stress the importance of an accurate and prompt identification of this rare form of AML, and suggest that, even in patients with isolated MS, the early administration of AML-like intensive chemotherapy followed by bone marrow transplantation might reduce the risk of subsequently developing systemic disease.