IgG subclasses of anti-Sm and anti-U1 ribonucleoprotein (U1 RNP) antibodies were determined using a new clone of the anti-IgG2 antibody (HG2-56F). Although the predominance of IgG1 coincided with previous reports, IgG2 anti-Sm and U1 RNP antibodies were detected in numerous patients. IgG3 anti-Sm antibody significantly correlates with joint involvement and a high titer of anti-DNA antibody. On the other hand, IgG4 anti-U1 RNP antibody significantly correlated with esophageal dilation and muscular involvement. These results may suggest that some IgG subclasses are related to a specific clinical feature or manifestation.