Sixty patients with adult acute lymphocytic leukemia (ALL) and 40 patients with blastic crisis of chronic myeloid leukemia (CML . BC) were randomly allocated by envelope method to receive either vindesine (VDS; 2 mg/m2 i.v., twice or once weekly) or vincristine (VCR; 1.2 mg/m2 i.v., once weekly) in combination with prednisolone (Pred; 40 mg/m2 p.o., daily). Out of 100 patients entered, 53 patients with ALL and 34 patients with CML . BC were evaluable. Remissions for ALL were seen in 20 of 25 patients (80.0%; 12 CRs and 8 PRs) treated with VDS regimen, and in 17 of 28 patients (60.7%; 7 CRs and 10 PRs) treated with VCR regimen, respectively. For CML . BC, however, remission rates for VDS regimen (42.1%; 5 CRs and 3 PRs in 19 patients) and for VCR regimen (40.0%; 2 CRs and 4 PRs in 15 patients) were equivalent. Overall remission rates of VDS regimen for both diseases, twice weekly regimen (65.5%; 11 CRs and 8 PRs in 29 patients) and once weekly regimen (60.0%; 6 CRs and 3 PRs in 15 patients) were similar. Patients treated with either regimen experienced high incidence of neurotoxicity. Neurotoxic disturbance appeared severer in patients treated with VCR regimen. Incidence of leukopenia and alopecia was high in patients treated with VDS regimen. These data suggest that VDS in combination with Pred is comparable-rather superior-to VCR in combination with Pred for adult ALL and CML . BC. Less neurotoxic and sufficiently effective dosage of VDS is considered to be 2 mg/m2, once weekly.