The dysregulation of cytokines is thought to play a role in the inflammatory and allospecific components of graft-versus-host disease (GVHD) and graft rejection (GR) post allogeneic bone marrow transplantation (BMT). Both complications occur post unrelated BMT at significantly higher frequencies than post BMT from identical sibling donors. The levels of tumor necrosis factor alpha (TNF-alpha) and interleukin (IL)-1beta, IL-6, IL-10 and IL-12 following unrelated BMT were measured to examine cytokine dysregulation involvement in GVHD and GR. Of the 26 patients included in this study, 15 developed GVHD (14 acute; 1 acute and chronic), 5 had GR and 6 had uneventful BMT. TNF-alpha was markedly elevated in 13/15 of the patients with GVHD and in 3/5 patients with GR. IL-6 was elevated in patients with acute GVHD and with GR. IL-12 levels were similar in pre- and post-BMT sera. No correlation was found between HLA-C match and cytokine levels. In conclusion, a positive correlation was found between elevated levels of TNF-alpha, IL-6 and IL-10, and GVHD (p < 0.05, p < 0.005 and p < 0.002 respectively) and GR (p < 0.01).