Imatinib Mesylate After a Donor Stem Cell Transplant in Treating Patients With Philadelphia Chromosome-Positive Leukemia

DISEASE CHARACTERISTICS: * Diagnosis of 1 of the following: * Acute lymphoblastic leukemia or chronic myeloid leukemia (CML) characterized by p\^190 and/or p\^210 bcr/abl gene rearrangement * Accelerated or blastic phase CML * CML in second or greater chronic phase * No imatinib mesylate-resistant leukemia * Planned allogeneic hematopoietic stem cell transplantation * Availability of an appropriately matched related or unrelated donor * Autologous or nonmyeloablative transplantation is not allowed * None of the following within 4 days after the date of neutrophil engraftment\*: * More than 5% marrow blasts * Circulating peripheral blood leukemic blasts * Aberrant antigen expression on marrow myeloblasts ≥ 1% by multidimensional flow cytometric assay * Presence of bcr/abl in \> 5% of marrow interphase nuclei by fluorescent in situ hybridization * More than 1 of 20 Philadelphia chromosome-positive marrow metaphases * CNS involvement by leukemia NOTE: \*The date of neutrophil engraftment is defined as the second consecutive day at which the peripheral blood absolute neutrophil count exceeds 500/mm3 PATIENT CHARACTERISTICS: Performance status * Not specified Life expectancy * At least 2 months Hematopoietic * See Disease Characteristics * Absolute neutrophil count ≥ 1,200/mm\^3 (use of filgrastim \[G-CSF\] allowed) Hepatic * Not specified Renal * Not specified Other * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception * No known imatinib mesylate hypersensitivity * No other disease that severely limits life expectancy PRIOR CONCURRENT THERAPY: Biologic therapy * See Disease Characteristics