The efficacy of abatacept, a drug that blocks T cell co-stimulation, has been investigated in a randomized, double-blind, placebo-controlled, phase III trial involving patients with psoriatic arthritis (PsA). The investigators randomly assigned patients with PsA to receive either 125 mg of abatacept weekly or placebo for 24 weeks. The ACR20 response rate at 24 weeks was higher in the abatacept group compared with the placebo group (39.4% versus 22.3%, respectively), whereas there was no difference between the groups in the incidence of adverse events. Treatment with abatacept resulted in a modest improvement of skin lesions.