Researchers compared 136 women with rheumatoid arthritis with 544 women of a similar age without the condition.
They found that women who had breastfed for 13 months or more were half as likely to get RA as those who had never breastfed. Those who had breastfed for one to 12 months were 25% less likely to get the condition, said the authors in the online edition of Annals of the Rheumatic Diseases.
But they also found that taking oral contraceptives, previously suspected of protect against RA because they contain hormones that are raised in pregnancy, did not have the same effect. Having children but not breastfeeding also did not seem to be protective, the authors said.