This study consists of 52 patients whose blood and bone marrow findings fulfilled the French-American-British (FAB) Cooperative Group criteria for a diagnosis of refractory anemia with excess of blasts in transformation (RAEB-T). The basis for the diagnosis of RAEB-T included 37 patients (71%) meeting one criterion, 13 patients (25%) meeting two criteria, and two patients (4%) meeting three criteria; 44% met only the criterion of the detection of Auer rods. Clonal chromosome abnormalities were seen in seven of 16 cases studied and included abnormalities of chromosome 5, an extra 8, and complex abnormalities. Thirty-one patients were treated with a chemotherapeutic regimen that included an anthracycline and cytarabine; 13 of 17 patients (77%) 45 years of age or younger and six of 14 patients (43%) over 45 years of age achieved CR. The median survival for the 17 patients 45 years of age or younger is 25+ months; the median survival for the 14 patients over 45 years of age is 12.3 months. The median survival for 13 untreated patients over the age of 45 is 4.5 months. The process in 18 of the 52 patients (35%) evolved to overt acute myeloid leukemia (AML) in 1 to 38 months after diagnosis of RAEB-T, in seven of 20 patients (35%) 45 years of age or younger, and 11 of 32 patients (34%) over 45 years. Fifteen of the 18 patients received antileukemic therapy in the RAEB-T phase. There were no clinical or laboratory features that were reliably predictive of transformation to AML.