The serum levels of interferon (IFN) were measured in 31 patients with systemic lupus erythematosus (SLE). Among the 31 patients, 8 (25%) showed IFN in the sera: in 4 patients IFN titers of 16 U/ml, and in the other 4 patients titers of greater than or equal to 32 U/ml became detectable. Whereas antibodies to native DNA (nDNA) were present in 6 of the 8 IFN positive patients, anti-nDNA antibodies were only positive in 6 of the 23 IFN negative individuals (p less than 0.002). Eight months after the first search for IFN in the patients' sera, 6 of the 8 IFN positive patients showed reduced IFN serum levels and one each had an increased or stable IFN titer. A good correlation between IFN titers and both the anti-nDNA antibody titers and the disease activity was observed. However, there were patients with active disease, high anti-nDNA titers but no IFN in the serum.