In 31 consecutive patients who received an allogeneic bone marrow transplantation the loss of proteins during the period at risk for acute graft-versus-host disease (aGVHD) was studied in order to determine whether the quantity of protein loss could be used for grading the severity of aGVHD. It was shown that the grade classified on the basis of the severity of skin rash, the quantity of diarrhea and the seriousness of cholestasis, correlated with serum albumin loss, intestinal plasma loss (expressed by the intestinal alpha 1-antitrypsin clearance) and the occurrence of inflammatory cells (leukocytes) in feces. The quantity of albumin lost by intestinal route accounted for only one third of the total albumin loss. To investigate whether the remaining part of it could be explained by capillary leakage elsewhere in the body, leakage of antileukoprotease from the tissue of the respiratory tract into the blood was measured. It was shown that the serum concentration of this proteinase inhibitor correlated with albumin loss. This means that capillary leakage also occurs in the lung during aGVHD. In conclusion, the loss of proteins can be used as a parameter of the severity of aGVHD once the proper diagnosis has been established. It appears that a combination of the current 'familiar' grading system and SAL yields a more objective classification system with a greater prognostic value.