Two patients developed antinuclear antibodies (ANA) including antibodies directed against double-stranded DNA (dsDNA) after bone marrow transplantation. In one of the patients very high levels of IgM, IgG as well as IgA anti-dsDNA were found during 6 months in the absence of symptoms of systemic lupus erythematosus (SLE). The other patient made IgM anti-dsDNA antibodies for a period of more than 3 years, also in the absence of SLE symptoms. The ANA were restricted to the IgG1 and IgG3 isotypes as is the case in SLE but did not express idiotypes commonly found in this autoimmune disease. The occurrence of these antibodies may reflect a non-antigen induced expansion of the existing donor B-lymphocyte repertoire.