Serum IgA levels were monitored at 3, 6, 12, and 24 months after BMT in 131 allogeneic and 3 syngeneic bone marrow transplant recipients. In general, IgA levels were low during the first 6 months and did not return to normal levels until 1-2 years after transplantation. Children (less than 15 years) had lower IgA levels at 3 and 6 months post-BMT compared to the adults (P less than 0.05), but donor age had no influence on the recipient IgA levels after BMT. Patients receiving either methotrexate or cyclosporine alone for GVHD prophylaxis had markedly lower IgA levels compared to those given a combination of these two drugs or patients transplanted with T-cell-depleted marrow (P less than 0.001). Mean IgA levels in patients without or with grade I acute GVHD were within the normal range at 3, 6, 12, and 24 months after BMT (greater than 0.3 g/L), although approximately 20% of the patients in each group showed low IgA levels (less than or equal to 0.3 g/L) early after transplantation. Patients with grade II or III acute GVHD had significantly lower values from 3 months up to 2 years after transplantation (P less than 0.01). Patients with chronic GVHD had significantly lower IgA levels 1 and 2 years after BMT compared to patients without chronic GVHD (P less than 0.005). Severe acute GVHD, particularly when followed by chronic GVHD, seems to be the main reason for low IgA levels, while other factors such as CMV infection or donor status may also contribute to the development of IgA deficiency after BMT.