AIMS: Serum concentrations of tumour necrosis factor-alpha (TNF) were determined in beta thalassemic patients before and after bone marrow transplantation (BMT) to evaluate whether changes in TNF concentrations after BMT were related to immune mediated complications. METHODS: Serum TNF concentrations were determined by enzyme linked immunoassay (EIA) in paired samples from 71 patients with beta thalassemia before and after BMT. Serial samples from 13 patients were also studied for up to six months after BMT. Forty one normal healthy children matched for sex and age were studied as controls. RESULTS: beta thalassemic patients had high serum TNF concentrations before transplantation compared with controls. These were not related to sex, age, duration of disease, number of blood transfusions, transferrin concentrations or splenectomy. DQw1 positive patients showed significantly lower TNF concentrations than non-DQw1 cases. Patients with severe liver fibrosis had significantly higher TNF concentrations. No correlation was found between TNF values and BMT outcome before transplantation but TNF alpha values fell significantly after BMT. The decrease persisted only in patients with successful engraftment. In serial samples studied for up to six months after BMT, TNF values decreased but in four out of five patients with graft rejection and in all five with acute graft versus host disease (GVHD) sharp increases occurred at the time of clinical symptoms. No correlation was found between the degree of GVHD and serum TNF-alpha concentrations nor between TNF-alpha concentrations after BMT and the presence of bacterial, viral, and fungal infections. CONCLUSIONS: About 50% of beta thalassemic patients have increased serum TNF, and the changes after BMT are related to the occurrence of immune mediate complications. The persistence of low TNF concentrations after successful engraftment may be due to the preparative regimen and the lack of adverse immune reactions.