Objective To analyse whether early arthritis patients who do not fulfil the American College of Rheumatology/European League Against Rheumatism (ACR/EULAR) 2010 classification criteria for rheumatoid arthritis (RA) have a different course of the disease dependent on whether they can or cannot be classified as RA because of radiographic disease (EULAR task force) at diagnosis. Methods For this observational study within the Swiss RA cohort SCQM, we included patients with early undifferentiated arthritis (disease duration ≤1 year), who had not received any previous disease-modifying antirheumatic drugs (DMARDs). 2010 ACR/EULAR criteria negative patients were separated into two groups (radiographic vs non-radiographic arthritis) depending on whether or not they had radiographic changes defined as erosive disease by a EULAR task force (≥3 joints with erosions). The primary outcome measure was the radiographic progression detected employing the Ratingen erosion score. Health Assessment Questionnaire (HAQ) and DAS-28 were used as secondary outcome measures. The average observation period was 4 years. Results A total of 592 patients were analysed. 240 were not classifiable as RA by application of the 2010 ACR/EULAR criteria at baseline. In 57 patients, radiographs at the first visit were not available. 133 patients had radiographic arthritis and 50 non-radiographic arthritis. Treatment was initiated in all patients with DMARDs, mostly methotrexate. No differences in DAS-28 and HAQ scores were found during follow-up. The average erosion scores were higher among patients with initially radiographic arthritis throughout the study. The progression of erosion scores over time, however, was higher in patients with initially non-radiographic arthritis with less subsequent radiological progression (3.3 erosions/year vs 0.4, respectively, p<0.0001). Conclusions The clinical and radiographic course of early undifferentiated arthritis under treatment was not dependent on the presence of erosions in three or more joints (ie, the definition of radiographic disease by the EULAR task force) at diagnosis in our cohort.