To describe (1) the findings with MRI in a series of patients with early undifferentiated oligoarthritis of the knee joint(s) and (2) the early effect after 2 months of treatment with only methotrexate (MTX) and hydroxychloroquine (HCQ) as disease-modifying antirheumatic drugs (DMARDs), 15 consecutive patients with undifferentiated oligoarthritis of the knee joint(s) were recruited. The mean age was 31.7 years (SD = 8.1 years), and the mean disease duration was 15.3 months (SD = 12.2 months). In all patients, synovial fluid analysis, RF, anti-CCP2 antibodies, ANA, CRP, ESR, and routine laboratory investigations were performed. Enhanced MRI was done at initial evaluation and after 2 months treatment. Four of the 15 patients had positive RF and 6 had positive anti-CCP2. After treatment with DMARDs, a regression was seen regarding effusion and synovitis in all patients; in one of three patients, the bone edema had regressed. Synovial thickening as measured by enhanced MRI decreased significantly (p < 0.01) and correlated significantly with the improved ESR and CRP (p < 0.01). After 2 months treatment with MTX and HCQ, the MRI improved considerably especially regarding synovial thickening.