Gemcitabine in Treating Children With Relapsed or Refractory Acute Lymphoblastic Leukemia or Acute Myelogenous Leukemia

DISEASE CHARACTERISTICS: * Diagnosis of relapsed acute lymphoblastic leukemia or acute myelogenous leukemia * M3 marrow (at least 25% blasts in bone marrow aspirate) * Refractory to conventional therapy PATIENT CHARACTERISTICS: Age: * 21 and under at diagnosis Performance status: * ECOG 0-2 OR * Zubrod 0-2 Life expectancy: * At least 2 months Hematopoietic: * Not specified Hepatic: * Bilirubin normal * SGOT or SGPT no greater than 2.5 times upper limit of normal Renal: * Creatinine normal OR * Creatinine clearance or radioisotope glomerular filtration rate at least 70 mL/min Other: * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception PRIOR CONCURRENT THERAPY: Biologic therapy: * No concurrent immunomodulating agents Chemotherapy: * At least 2 weeks since prior chemotherapy * No other concurrent chemotherapy for cancer Endocrine therapy: * No concurrent corticosteroids except for treatment of adrenal crises with suppressed pituitary/adrenal response * Concurrent low-dose hydrocortisone (less than 100 mg/m2) allowed for allergic reactions to amphotericin or transfusions Radiotherapy: * Concurrent radiotherapy to localized painful lesions allowed Surgery: * Not specified Other: * Recovered from any prior therapy