A proof-of-concept prospective, open-label study has shown that abatacept has promise as a treatment for patients with anti-citrullinated protein antibody (ACPA)-negative undifferentiated arthritis (UA) and poor prognosis (power Doppler ultrasonography (PDUS) score ≥1). The primary end point was a composite of DAS44 remission (DAS44 <1.6, where DAS44 is the disease activity score incorporating 44-joint swollen joint count, Ritchie Articular Index and erythrocyte sedimentation rate), a maximum of one swollen joint for ≥3 consecutive months, and no measured radiographic progression at 6 months. Two of the 20 (10%) patients treated with abatacept monotherapy for 6 months achieved the defined primary end point, and six (30%) achieved DAS44 remission (95% Cl 15–51%) by 6 months. By 12 months, 8/20 (40%) patients achieved DAS44 remission (95% CI 22–61%); the mean DAS44 was reduced from 2.66 to 1.78 and the median PDUS score reduced from 10 to 3 at this time point.