IgE levels were determined before and serially after allogeneic bone marrow transplantation (BMT) in 12 patients. Six patients had aplastic anemia, four leukemia, and one each Wiskott-Aldrich syndrome and infantile agranulocytosis. IgE levels increased sharply (7- to 2,000-fold) in 10 of the 12 as early as 14 days after BMT. They all returned to baseline levels by 60 days. In six of these patients, the rise accompanied clinical and biochemical evidence of acute graft-versus-host disease (GVHD). All of the patients who received rabbit antihuman thymocytic serum (ATS) in preparation for transplantation and were tested for IgE antirabbit serum antibody by radioallergosorbent test (RAST) (n = 6) developed a strongly positive RAST which paralleled their total IgE levels. These high IgE levels detected during the period of acute GVHD may be a manifestation of a transient lack of suppressor T cell activity.