Significance of antinuclear antibodies (ANA) in the patients with chronic fatigue syndrome (CFS) was reviewed. When indirect immunofluorescence with the HEp-2 cells as the substrates was used, prevalence of the positive ANA was reportedly 15-25%. The ANA titers were low and the immunofluorescent staining patterns were heterogeneous. One group in the USA reported that 'nuclear envelope staining pattern' was found in more than 50% of the patients with CFS. This results, however, have not been confirmed by any other research groups. Clinical significance of the positive ANA in the CFS patients resides in differential diagnoses of systemic lupus erythematosus and other diffuse connective tissue diseases. Recently, several ANAs specific to CFS have been described. We reported anti-68/48kD protein antibodies utilizing SDS-PAGE/ immunoblot method. These autoantibodies were found in 13% of 114 CFS patients and 0% in healthy subjects (p < 0.05). Hypersomnia and difficulty in concentration were found more frequently in the CFS patients with this specific autoantibody.