Before treatment, allergy to ATG was determined by intradermal injection of 0.1 mL of ATG at a concentration of 5 mg/mL. Antithymocyte globulin treatment was contraindicated if erythema larger than 5 mm in diameter, compared with the saline control, developed. Eligible patients were given ATG (Pharmacia, Peapack, New Jersey), 40 mg/kg of body weight daily for 4 days. Antithymocyte globulin was administered intravenously over 4 to 8 hours with oral prednisone, 1 mg/kg daily [minimum of 40 mg/d], for 10 days and then tapered by day 17. Clinic visits were scheduled at 3, 6, and 12 months and then annually. Bone marrow aspirate, biopsy specimens, and bone marrow samples for cytogenetic analysis were obtained at 6 and 12 months and then annually. Excluding transfusions and antibiotics, patients received no other treatment for myelodysplastic syndromes for at least 6 months. The main criterion for response was independence from red blood cell transfusion. This criterion was met if 1) the patient was independent from transfusion for a minimum of 6 weeks with a sustained increase in hemoglobin level, or 2) the patient maintained a stable hemoglobin level within 8 months of ATG treatment, as measured from the time the patient last received a transfusion until reinstitution of red blood cell transfusions. Standard criteria for red blood cell transfusion were followed. Transfusions were given for hemoglobin nadirs between 7 and 10 g/dL, depending on subjective tolerance of anemia in individual patients. Platelet and neutrophil counts that were the highest at 3 and 6 months of follow-up were used as the peak response. Thrombocytopenia was defined as a platelet count less than 150 × 109cells/L, and neutropenia was defined as a neutrophil count less than 1.0 × 109 cells/L. In patients with severe thrombocytopenia with a platelet count less than 20 × 109 cells/L, a response was defined as a sustained increase to at least 25 × 109 cells/L. In patients with severe neutropenia with a neutrophil count of 0.5 × 109 cells/L, a response was defined as an increase to 1.0 × 109 cells/L or greater. Morphologic characteristics of bone marrow, cellularity, and karyotype were compared before treatment, after 6 months, and then annually.