T cell costimulation is believed to be crucial in orchestrating immune responses that lead to inflammation and destruction in rheumatoid arthritis (RA). Abatacept is a novel recombinant CTLA4Ig fusion protein that selectively modulates costimulation via interrupting the CD28:CD80/86 pathway, resulting in downregulation of T cell activation and multiple ensuing effector mechanisms. Abatacept has been shown to be efficacious, either when given alone or in combination with methotrexate, in patients with active RA, including anti-TNF failures. Improvements in clinical signs and symptoms, slowing of radiological progression, and enhancement in patient function and pain have been reported in clinical trials. Infusions were well-tolerated with a favourable safety profile similar to placebo and no appreciable immunogenicity. Abatacept is the first in a new class of biological response modifiers called costimulatory blockers.