Determining the progression of anatomic involvement in rheumatoid arthritis (RA) is a well recognized problem. In 1976, Trentham and Masi described the carpometacarpal (CMC) ratio and found it useful for detecting progression of anatomic involvement in RA. Our study attempts to validate the efficacy of this ratio both in a cross sectional (n = 267) and longitudinal (n = 53) sample of patients with rheumatoid arthritis. Our data indicate that the CMC ratio is useful for quantitating anatomic involvement and correlates well with anatomic progression over time.