TNF alpha levels were determined by ELISA in serum from 112 BMT patients during pre-transplant conditioning. Patients who developed post-transplant complications had significantly higher TNF alpha levels than those without complications (mean 620 pg/ml vs 440 pg/ml, P = 0.04). In particular this effect is associated with patients who developed grade II-IV acute GVHD (mean 960 pg/ml, P < 0.001) and chronic GVHD (mean 724 pg/ml, P = 0.001). High TNF alpha levels were the only statistically significant risk factor for acute GVHD. IL-1 beta and IL-6 levels were not correlated with TNF alpha levels or posttransplantation complications. In multivariate analysis of chronic GVHD, patient age > 17 years and CMV disease were the only statistically significant risk factors. Relapse was associated with low levels of TNF alpha during conditioning (mean 318 pg/ml, P = 0.02). In multivariate analysis, high risk disease was the only factor that correlated with relapse. Low risk patients had significantly higher levels than high risk patients (551 vs 377, P= 0.04). CML and MDS patients had higher TNF alpha levels than acute leukemia patients. There was no difference in TNF alpha levels between patients conditioned with BU/CY and CY/TBI. We conclude that determination of TNF alpha levels during conditioning may be useful in the prediction of acute GVHD.