In this prospective study of 24 pregnant patients with rheumatoid arthritis, quiescent disease activity in 21 patients (88%) during gestation was followed by more active disease in the puerperium in 19 patients (79%). Increased disease activity was reflected in a deterioration in manual dexterity and this was found to correlate with a post-partum rise in IgM rheumatoid factor (IgM RF) (r = 0.86) and a post-partum decline in pregnancy associated alpha-2 glycoprotein (PAG) (r = 0.44). The increase in IgM RF also correlated with increased disease activity measured by a visual analogue scale (r = 0.44). Changes in IgA RF were not observed. These results suggest that PAG and IgM RF could contribute to the modulation and pathogenesis of rheumatoid arthritis during pregnancy.