DISEASE CHARACTERISTICS: Histologically proven relapsed acute lymphocytic leukemia, acute myeloid leukemia, or blastic phase chronic myelogenous leukemia Refractory to conventional therapy and other therapies of higher priority May have concurrent extramedullary relapse except for testicular relapse or other extramedullary sites that may require concurrent radiotherapy

PATIENT CHARACTERISTICS: Age: 21 and under Performance status: ECOG 0-2 Life expectancy: At least 2 months Hematopoietic: Not specified Hepatic: Bilirubin no greater than 2.0 times normal SGOT or SGPT less than 5 times normal Renal: Creatinine no greater than 1.5 times normal Other: Able to take oral liquid medication No GI neuropathy No other condition that may affect absorption of drug No diabetes mellitus Not pregnant or nursing Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: Prior bone marrow transplantation (BMT) or peripheral blood stem cell transplantation (PBSCT) allowed and recovered At least 2 weeks since prior cytokine therapy and recovered No concurrent immune modulator therapy No concurrent cytokines including interleukin-11, interleukin-2, and epoetin alfa Chemotherapy: At least 2 weeks since prior chemotherapy (4 weeks for nitrosoureas) and recovered No more than 3 prior chemotherapy regimens No other concurrent chemotherapy Endocrine therapy: No concurrent steroids Radiotherapy: No prior craniospinal radiotherapy Prior total body irradiation allowed as part of BMT or PBSCT and recovered Concurrent radiotherapy for localized painful lesions allowed Surgery: Not specified Other: No concurrent metoclopramide or cisapride to maintain motility or gastric emptying No concurrent H2 antagonists No concurrent proton pump inhibitors No concurrent antacids for gastritis, gastroesophageal reflux, or ulcers (gastric or duodenal) No antacid therapy for 6 hours before and for 90 minutes after topotecan administration