Hidden rheumatoid factor (HRF) was detected in rheumatoid arthritis (RA) serum with ELISA after separation and dissociation of immune complex (IC). It was found that 60% of the patients with active RA has positive latex fixation test, whereas 96% of them has been demonstrated to have IgM hidden rheumatoid factor (HIgMRF) and 70% of them IgG hidden rheumatoid factor (HIgGRF). When the disease was controlled, HIgMRF and HIgGRF was found in 24% and 12% respectively. The difference was significant with P < 0.01. Our results show that the presence of HRF in RA patients is related with the activity of the disease, but not with duration of disease, sex and age.