Serum IgE levels were studied in 25 bone marrow transplant recipients (in 12 patients twice weekly and in 13 patients, at random). A 2-748-fold increase in serum IgE was recorded in 20 of the 25 patients after transplantation, the highest IgE value observed being 8,000 kU/liter. The IgE elevation appeared concomitantly with acute graft-versus-host disease (GVHD) in 14 patients. Both events occurred on day 24 +/- 2 (mean +/- SE). When the acute GVHD was diagnosed, there was a significant increase in serum IgE as compared to the first posttransplantation value. In one patient in whom GVHD recurred, a second IgE peak was seen, and in another patient with flaring GVHD, IgE levels increased on several occasions. In 6 patients without clinical signs of GVHD, a rise in IgE occurred on day 35 +/- 12. One of these patients was grafted with marrow from her identical twin. The rise in IgE did not correlate with an elevated proportion of eosinophil granulocytes. In the majority of the patients, no correspondent increases in serum IgG, IgA, or IgM were seen during the period with increased IgE after transplantation.