The diagnostic significance of anti-double-stranded deoxyribonucleic acid (anti-dsDNA) determination was evaluated in a prospective manner from 1974 to 1982 in a group of 441 patients without systemic lupus erythematosus whose sera were found to contain antibodies to dsDNA on routine screening (Farr assay). Within one year 69% (304) of these patients fulfilled the preliminary American Rheumatism Association (ARA) criteria for systemic lupus erythematosus (SLE). Eighty-two of the remaining 137 patients were followed up for several years. At the end of the study 52% of these patients had also developed systemic lupus erythematosus. Patients who developed systemic lupus erythematosus were characterised by the occurrence of relatively high avidity anti-dsDNA in the circulation compared with patients who did not develop systemic lupus erythematosus. It can be concluded that about 85% of patients without systemic lupus erythematosus with anti-dsDNA in the circulation will develop SLE within a few years. Taking into account the relative avidity of anti-dsDNA, as determined by calculation of Farr/polyethylene glycol (PEG) ratios, we conclude that patients with relatively high avidity anti-dsDNA are more prone to develop systemic lupus erythematosus than patients with relatively low avidity anti-dsDNA.