The present study sought to retrospectively assess risk factors for occurence of and mortality from severe acute graft-versus-host-disease (aGvHD) among 66 patients receiving hematopoietic stem cell transplants (HSCT) from matched sibling donors (MSD). Thirty-six patients were in early and 30 in intermediate or advanced stages of the disease. Twenty-six patients developed severe aGvHD grades II-IV. Thirty-five patients died after transplantation (15 due to aGvHD). There were 20 major ABO-mismatched transplants and 26 cases wherein donor and recipient differed with respect to sex (11: F-->M; 15: M-->F). These transplant characteristics as well as HLA class II specificities were chosen for discriminative analysis. HLA specificities were assessed in an independent analyses for as patients lacking (DR11) or having DR13 associated with aGvHD. It appeared that donor age increased the risk of aGvHD, but a fatal outcome of this complication was influenced by recipient age. Female to male transplantations were associated with a higher risk of aGvHD. Major ABO incompatibility tended to increase the risk of aGvHD and fatal outcomes. DR11 was associated with factors playing a protective role, while DR13 was the least of all significant factors influencing the development of severe and fatal aGvHD.