Antithymocyte Globulin Compared With Supportive Care in Treating Patients With Myelodysplastic Syndrome

DISEASE CHARACTERISTICS: * Histologically or cytologically confirmed early myelodysplastic syndrome (MDS) with less than 10% bone marrow blasts * Refractory anemia (RA) * RA with excess blasts (RAEB) * Hypocellular myelodysplasia * Low or intermediate-1 prognostic risk * Transfusion-dependent * Need for 2 or more units of red blood cells or platelets per month for 2 or more months prior to study OR * History of prior transfusions and 2 consecutive (at least 21 days apart) hemoglobin levels less than 8.0 g/dL or platelet counts less than 20,000/mm\^3 during the past 2 months * Hemoglobin no greater than 12.0 g/dL after prior transfusion * No myelosclerosis occupying more than 30% of bone marrow space * No RA with ringed sideroblasts, RAEB in transformation, or chronic myelomonocytic leukemia * No therapy-related MDS * No history of immune-related hematologic disorder (e.g., idiopathic thrombocytopenic purpura) PATIENT CHARACTERISTICS: Age: * 18 and over Performance status: * ECOG 0-2 Life expectancy: * At least 3 months Hematopoietic: * See Disease Characteristics * No other causes of cytopenia unrelated to MDS (e.g., gastrointestinal blood loss) * Iron present on marrow examination OR * Transferrin saturation at least 20% and ferritin at least 50 ng/mL Hepatic: * Bilirubin no greater than 2 mg/dL OR * SGOT/SGPT no greater than 2 times normal * No active or chronic hepatitis B or C Renal: * Creatinine no greater than 2 mg/dL Cardiovascular: * No symptomatic cardiac disease * No congestive heart failure (even if medically controlled) * No myocardial infarction within the past 6 months Pulmonary: * No severe pulmonary disease * If history of pulmonary insufficiency, must have pO\_2 at least 90 mm/Hg on room air or pCO\_2 no greater than 40 mm/Hg Other: * No history of unresolved B12 or folate deficiency since diagnosis of MDS * No untreated acute or chronic infection (afebrile for 7 days without antibiotics prior to study) * No active or chronic HIV * No concurrent cytomegalovirus infection * No other malignancy within the past 2 years except adequately treated localized squamous cell or basal cell skin cancer or carcinoma in situ of the cervix * No concurrent drug or alcohol abuse * No significant medical or psychosocial problems * No known allergy to rabbit protein * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception PRIOR CONCURRENT THERAPY: Biologic therapy: * At least 8 weeks since prior biologic agents, colony-stimulating factors, or epoetin alfa for MDS * At least 8 weeks since other prior investigational biologic agents * No prior or concurrent bone marrow transplantation * No concurrent epoetin alfa * No concurrent growth factors except filgrastim (G-CSF) or sargramostim (GM-CSF) for neutropenic fevers * No other concurrent biologic agents Chemotherapy: * At least 8 weeks since prior cytotoxic drugs for MDS * Concurrent chemotherapy for clinical indications of disease progression or leukemic transformation allowed Endocrine therapy: * At least 8 weeks since prior androgenic hormonal therapy for MDS * At least 8 weeks since prior danazol for MDS Radiotherapy: * No prior radiotherapy Surgery: * No prior organ transplantation Other: * At least 8 weeks since prior investigational drugs * At least 8 weeks since prior immunosuppressive drugs or other drugs for MDS * No concurrent immunosuppressive therapy * No other concurrent experimental drugs