Overall activity of classical complement pathway, C3 and C4 levels, level of circulating immune complexes and concentration of serum immunoglobulins were measured in 38 patients transplanted with HLA-identical bone marrow before and after transplantation for at least 4 years. Changes of complement parameters and their association with acute and chronic GVHD and with infections were analysed. A strong association was found between the development of chronic GVHD and hypercomplementaemia measured in 16 long-term survivors. Low pre-transplantation C4 activity was found to predict the development of severe acute GVHD. These findings indicate that longitudinal complement measurements may have clinical value in BMT patients.