Rheumatoid arthritis (RA) is an autoimmune inflammatory disease characterized by bone loss. Degree of inflammation and autoantibody positivity have both been identified as important initiators of skeletal damage in RA. Whereas it is well appreciated that inflammation initiates bone loss via the action of cytokines, the effect of autoantibodies in initiating bone destruction has long been underestimated. It has, nonetheless, been known for years that antibodies against citrullinated proteins (ACPA) and rheumatoid factor are associated with a more destructive disease course. It was recently shown that ACPA bind osteoclast precursor cells and directly promote their differentiation into bone-resorbing osteoclasts. Other results have shown that in ACPA-positive individuals bone loss starts even before the onset of clinical disease; this is indicative of the independent effect of these antibodies in initiating skeletal damage. The observation that the mere presence of ACPA is associated with pathological changes suggests that these antibodies have functional properties and initiate the onset of disease long before patients consult the rheumatologist because of symptomatic joint disease. These findings also indicate that "RA" is a syndrome rather than a single disease, suggesting that autoantibody-positive patients are both genetically and clinically distinct phenotypes.