We studied serum amyloid A levels of 31 bone marrow transplant recipients with and without acute graft-versus-host disease. Before transplantation the mean SAA concentration was 5.1 +/- 0.8 mg/l (mean +/- SEM). It remained low in patients with no signs of aGVHD but increased significantly during aGVHD to 54.0 +/- 8.2 mg/l (p less than 0.001 compared to pre-BMT value). Severe infections also induced high SAA levels.