We have previously reported that about 80% of acute myeloid leukaemia (AML) samples tested at diagnosis constitutively expressed cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4). The present study compared CTLA-4 expression and function of leukaemic cells from AML patients at diagnosis with those from AML patients resistant to conventional chemotherapy. We also explored the possibility of targeting CTLA-4 for apoptosis induction in chemoresistant AML cells. AML cells either from untreated patients (n = 15) or in chemoresistant phase (n = 10) were analysed for CTLA-4 protein and transcript expression by flow cytometry and reverse transcription-polymerase chain reaction respectively. CTLA-4 expression was similar in untreated and in chemoresistant samples and was not associated with patients' clinical features. In chemoresistant AML cells, CTLA-4 transduced an apoptotic signal on engagement with its recombinant ligands r-CD80 and r-CD86, which induced an average of 71% and 62% apoptotic cells, respectively, at highest concentration. Apoptosis was equally induced in untreated leukaemic cells accompanied by cleavage of procaspase-8 and -3. Thus, this study provides the first evidence that killing of leukaemic cells from AML patients may be obtained by the engagement of CTLA-4 with its ligands, opening the way to a novel potential therapeutic approach based on triggering the CTLA-4 molecule to circumvent chemoresistance in AML.