S0521, Combination Chemotherapy With or Without Gemtuzumab Followed By Tretinoin, Mercaptopurine, and Methotrexate or Observation in Treating Patients With Acute Promyelocytic Leukemia

DISEASE CHARACTERISTICS: * Cytologically confirmed acute promyelocytic leukemia (APL) or the variant form of APL * Previously untreated disease * Low- or intermediate-risk disease * Low-risk disease, defined as white blood cell (WBC) ≤ 10,000/mm\^3 and platelet count \> 40,000/mm\^3 * Intermediate-risk disease, defined as WBC ≤ 10,000/mm\^3 and platelet count ≤ 40,000/mm\^3 * WBC and platelet count confirming low- or intermediate-risk disease must be obtained within 48 hours prior to study registration unless the patient received tretinoin therapy prior to study registration in which case the WBC and platelet count must be obtained within 48 hours prior to study therapy * PML-RARα fusion gene positive by reverse transcriptase-polymerase chain reaction (RT-PCR) assay * No recurrent disease * Must be registered on clinical trials SWOG-9007 and SWOG-S9910 * Specimens must be collected prior to tretinoin therapy and may be collected after tretinoin therapy PATIENT CHARACTERISTICS: * Zubrod performance status 0-3 * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception for ≥ 1 month prior to, during, and for 2 months after completion of study treatment * No unstable cardiac arrhythmia or unstable angina * No other malignancy within the past 5 years except for the following: * Adequately treated basal cell or squamous cell skin cancer * Carcinoma in situ of the cervix * Adequately treated stage I or II ca…