After 2 years, only 47 (9.8%) of 277 patients with undifferentiated arthritis (UA) from the Norwegian Very Early Arthritis Clinic study went on to fulfil the 2010 ACR–EULAR criteria for rheumatoid arthritis (RA). Characteristics of patients with UA that progressed to RA include older age, female sex, ever smoking, rheumatoid factor or anticitrullinated protein antibody positivity and initial presentation with polyarticular arthritis with small-joint involvement and a swollen shoulder joint. Patients with UA that developed into RA were also more likely to use DMARDs than those with UA that did not develop into RA (P < 0.001).