A solid phase Enzyme-Linked Immunosorbent Assay (ELISA) was developed for the measurement of IgG and IgM antibodies to double-stranded DNA (anti-dsDNA). This method is sensitive, specific, relatively simple and suitable for routine use. Thus, we evaluated sera from 224 Greek patients with the following autoimmune rheumatic diseases: 54 patients with classical rheumatoid arthritis (RA), 50 patients with primary Sjögren's syndrome (SS), 41 patients with systemic lupus erythematosus (SLE), 30 patients with scleroderma, 20 patients with idiopathic Raynaud's phenomenon (IRP) and 29 patients with juvenile rheumatoid arthritis (JRA). Sera from 119 age- and sex-matched healthy blood donors were tested as normal controls. The presence of both IgG and IgM anti-dsDNA highly correlated with SLE. However, IgM anti-dsDNA levels were significantly lower. Serum complement C3 and C4 levels correlated negatively with anti-dsDNA levels in the SLE group. Finally, in sequential sera from five SLE patients, the anti-dsDNA activity proved to be a relatively sensitive marker of SLE activity.