We conducted a study to demonstrate vincristine-induced apoptosis in vivo in peripheral blood mononuclear cells of children with newly diagnosed acute lymphoblastic leukaemia (ALL). In five children, apoptosis was detected by terminal deoxynucleotide transferase-mediated dUTP-digoxigenin nick-end labelling (TUNEL) assay in blood samples collected during an up-front window study of vincristine monotherapy. In one patient, we found a striking increase of apoptotic cells from 2% (SEM 0.6) before to 40.7% (SEM 2.9) 3 h after vincristine injection. In the other patients, the maximum increase ranged from 0.8% to 4.3%. Wilcoxon matched pairs analysis of all patients confirmed that vincristine induces apoptosis in vivo in peripheral blood mononuclear cells in childhood ALL (P = 0.043).