Antinuclear antibodies react with various nuclear autoantigens. Testing for them is critical for diagnosing connective tissue diseases, but they may be also found in other inflammatory diseases and in the elderly. They constitute a group of heterogeneous autoantibodies, and their specificity helps to discriminate between various connective tissue diseases. Screening for total antinuclear antibodies is generally performed by indirect immunofluorescence on HEp2 cells. In the presence of antinuclear antibodies, anti-dsDNA antibodies and anti-ENA (extractable nuclear antigens) antibodies must be sought. Positivity for IgG anti-dsDNA antibodies is highly characteristic of systemic lupus. A rapid increase in their titer suggests visceral, particularly renal, involvement. Anti-Sm antibodies are also characteristic of systemic lupus, while anti-JO1 and anti-Scl70 antibodies suggest, respectively, myositis and diffuse systemic sclerosis. Antibody titers do not provide information about disease course. The presence of anti-Ro/SSA antibodies during pregnancy requires a workup for cardiac conduction disturbances in the fetus.