Osteoclasts (OC) are clearly involved in joint destruction in Rheumatoid Arthritis (RA), as shown by clinical and experimental data. Tumor Necrosis Factor Alpha (TNF-alpha), a major pathologic mediator in RA, may induce bone resorption either directly, stimulating osteoclastogenesis or indirectly, by influencing receptor activator of nuclear factor kappa-B ligand (RANKL), osteoprotegerin (OPG) and prostaglandin production by osteoblasts. Anti-TNF agents reduce bone destruction in RA but their effects on osteoclast biology in patients with RA are little known.