• The proportion of subjects who achieve remission as defined by a DAS 28 score less that 2.6. [ Time Frame: in 12 months of treatment (Day 365) ] [ Designated as safety issue: No ]
• Joint damage progression measured by radiographic evaluation using the Genant‑Modified Sharp total score [ Time Frame: at 12 months of treatment (Day 365) ] [ Designated as safety issue: No ]

• The proportion of subjects who achieve remission in 12 months of treatment (Day 365) as defined by a DAS 28 score less that 2.6.
• Joint damage progression measured by radiographic evaluation using the Genant‑Modified Sharp total score at 12 months of treatment (Day 365).

• Compare the proportion of subjects with an ACR50 response [ Time Frame: at month 12 (Day 365) ] [ Designated as safety issue: No ]
• Compare the proportion of subjects achieving major clinical response defined by 6 months of consecutive ACR 70 response [ Time Frame: at month 12 (Day 365) ] [ Designated as safety issue: No ]
• Compare the disease activity as measured by DAS 28 score [ Time Frame: at month 12 (Day 365) ] [ Designated as safety issue: No ]
• Compare the improvement in physical function using the HAQ disability index [ Time Frame: at month 12 (Day 365) ] [ Designated as safety issue: No ]
• Compare the improvement in health-related quality of life using SF-36 [ Time Frame: at month 12 (Day 365) ] [ Designated as safety issue: No ]
• Compare the inhibition of joint damage progression measured by radiographic evaluation using the Genant-modified Sharp erosion, and joint space narrowing [ Time Frame: at month 12 (Day 365) ] [ Designated as safety issue: No ]
• Assess the inhibition of joint damage progression measured by radiographic evaluation using the Genant-modified Sharp erosion, joint space narrowing and total score [ Time Frame: at month 24 (Day 729) ] [ Designated as safety issue: No ]
• Determine the safety and tolerability of abatacept in this subject population, including evaluation of immunogenicity of abatacept. [ Time Frame: throughout the study ] [ Designated as safety issue: Yes ]
• Assess the improvement in physical function [ Time Frame: at month 24 (Day 729) ] [ Designated as safety issue: No ]

• Compare the proportion of subjects with an ACR50 response at month 12 (Day 365).
• Compare the proportion of subjects achieving major clinical response defined by 6 months of consecutive ACR 70 response at month 12 (Day 365).
• Compare the disease activity as measured by DAS 28 score at month 12 (Day 365).
• Compare the improvement in physical function using the HAQ disability index at month 12 (Day 365) and assess the improvement in physical function at month 24 (Day 729).
• Compare the improvement in health-related quality of life using SF-36 at month 12 (Day 365).
• Compare the inhibition of joint damage progression measured by radiographic evaluation using the Genant-modified Sharp erosion, and joint space narrowing at month 12 (Day 365).
• Assess the inhibition of joint damage progression measured by radiographic evaluation using the Genant-modified Sharp erosion, joint space narrowing and total score at month 24 (Day 729).
• Determine the safety and tolerability of abatacept in this subject population, including evaluation of immunogenicity of abatacept.

This is a world wide study to evaluate the remission and joint damage in subjects treated with abatacept in addition to methotrexate versus subjects who receive methotrexate along with a placebo.

• Drug: abatacept and methotrexate
• Drug: placebo and methotrexate

Inclusion Criteria:

• Diagnosis of rheumatoid arthritis (RA) <2 years; never received treatment with methotrexate; erosions noted on x-ray.
• CRP >= 8.0 mg/L
• Rheumatoid factor or anti CCP positive
• Additional laboratory requirements

Exclusion Criteria:

• Women and men who are not willing to use birth control
• Diagnosed with other rheumatic disease
• History of cancer within 5 years
• Active tuberculosis
• Treatment with another investigation drug within 28 days
• Active bacterial or viral infection