Dear Sir, The development of factor VIII (FVIII) neutralizing antibodies (inhibitors) is one of the serious complications in the clinical management of haemophilia A. As replacement therapy with FVIII concentrates is ineffective or markedly impaired in this setting, the management of inhibitor patients is full of difficulty. Although FVIII inhibitor will develop by both B cell-and T celldependent immune system (1, 2), the precise immune regulatory mechanism of inhibitors has not been fully addressed. Recently, new ligands, BAFF (B cell-activating factor belonging to the tumour necrosis factor [TNF] family, also known as BlyS) and APRIL (a proliferation inducing ligand), have been found to regulate immune system. BAFF is a member of the TNF superfamily of ligands and is involved in the survival and maturation of B cells (3). Another member of the superfamily, APRIL, also stimulates B-and T-cell proliferation and triggers humoral immune responses (4). BAFF binds to the TNF-related receptors such as B-cell maturation antigen (BCMA), transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) and BAFF receptor (BAFFR), whereas APRIL binds to TACI and BCMA and to heparan-sulfate proteoglycans (5). Such BAFF/APRIL ligand-TNF related receptor interaction comprises a complex network that is critically involved in the induction and regulation of humoral immunity. We hypothesized that BAFF and/or APRIL would participate in the development and preservation of inhibitors. To test the hypothesis, we measured BAFF and APRIL levels in haemophilia A patients with and without inhibitors.