Given that aggressive DNA damaging chemotherapy shows suboptimal efficacy in chronic lymphocytic leukaemia (CLL), alternative therapeutic approaches are needed. Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) is able to induce tumour-specific apoptosis. However, apoptosis might be inhibited by elevated levels of X-linked inhibitor of apoptosis (XIAP). Use of XIAP-inhibiting compounds might sensitize primary CLL cells towards TRAIL-mediated apoptosis. A novel small molecule, compound A (CA), an inhibitor of XIAP, was used in combination with TRAIL to induce apoptosis in primary CLL cells (n = 48). XIAP was significantly more highly expressed in primary CLL cells (n = 28) compared to healthy B cells (n = 16) (P = 0·02). Our data obtained by specific knock-down of XIAP by siRNA identified XIAP as the key factor conferring resistance to TRAIL in CLL. Combined treatment with CA/TRAIL significantly increased apoptosis compared to untreated (P = 8·5 × 10−10), solely CA (P = 4·1 × 10−12) or TRAIL treated (P = 4·8 × 10−10) CLL cells. CA rendered 40 of 48 (83·3%) primary CLL samples susceptible to TRAIL-mediated apoptosis. In particular, cells derived from patients with poor prognosis CLL (ZAP-70+, IGHV unmutated, 17p-) were highly responsive to this drug combination. Our highly-effective XIAP inhibitor CA, in concert with TRAIL, shows potential for the treatment of CLL cases with poor prognosis and therefore warrants further clinical investigation.