Summary. We have studied the actions of tumour-necrosis-factor-related apoptosis-inducing ligand (TRAIL) on cells isolated from patients with acute myeloid leukaemia (AML). Apoptosis induction was initially assessed by quantitative morphological analysis. Only 2/19 isolates showed a > 10% increase in apoptotic cells following TRAIL treatment. However, incubation with TRAIL combined with fludarabine, cytosine arabinoside or daunorubicin resulted in additive or super-additive apoptosis induction in approximately half of the isolates. Molecular evidence of super-additive apoptosis induction by TRAIL and cytotoxic agents was obtained by quantification of caspase 3 activation, detected by Western blot analysis of poly (ADP ribose) polymerase cleavage. The ability of TRAIL and daunorubicin to induce super-additive apoptosis correlated with the ability of these agents to activate caspase 8 and to augment cellular levels of the truncated pro-apoptotic form of the BCL-2 family member BID. Our data suggest that co-administration of TRAIL with conventional cytotoxic drugs may be of therapeutic value in some patients with AML.