OBJECTIVE: Antiperinuclear factor (APF)is regarded as a marker antibody with high sensitivity and specificity for rheumatoid arthritis (RA).However, the precise role of APF in the pathogenesis of RA has not yet been elucidated. Studying the isotype of APF may be a step in revealing the nature of this anti-body,but such studies have been rare,and none have been done in Korea.The present study is set out to identify the isotype of APF from sera of Korean patients with RA,and to determine whether certain isotypes are related to specific clinical aspects. METHODS: A total of 114 APF positive RA sera were tested against IgG,IgA and IgM separately by indirect immunofluorescence method,and the medical records were retrospectively reviewed. RESULTS: All 114 serum samples were positive for IgG-APF,100 positive for only IgG-APF (G group,87.7%),9 positive for IgG and IgA-APF (GA group, 7.9%),4 positive for IgG and IgM-APF (GM group,3.5%),and one sample was positive for IgG,IgA and IgM-APF (0.9%).The levels of rheumatoid factor (RF)and C-reactive protein (CRP)were different between the 3 groups (ANOVA test),and the RF and CRP of the GM group was higher than the other 2 groups (Bonferroni test). CONCLUSION: The APF identified in Korean patients with RA was always IgG-APF.IgM-APF may be used as a serological marker to assess disease activity in conjunction with CRP.The differences in the laboratory parameters between each isotype groups indicate the possibility of utilizing the isotype of APF to determine the disease activity or prognosis of RA.