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In early rheumatoid arthritis (RA), proliferative synovitis sometimes occurs earlier in the tenosynovium or bursal synovium than in the articular synovium. Here we report two patients who presented with subcalcaneal bursitis while progressing from undifferentiated arthritis with high-titer anti-CCP antibodies (ACPA) to a diagnosis of RA. They had initially presented with palindromic transient pain in the hands and the feet. They were strongly positive for ACPA and negative for rheumatoid factor (RF) at the onset of symptoms. A few years later, they developed persistent plantar heel pain and underwent musculoskeletal ultrasonography (MSUS). MSUS revealed subcalcaneal bursitis with synovial proliferation. At that time, they became positive for RF and they were clinically diagnosed and began receiving treatment for RA. They developed overt synovitis in their wrists and fingers several months later. To the best of our knowledge, this is the first report on MSUS-detection of subcalcaneal bursitis with synovial proliferation in patients in the very early phase of RA, although there have been many reports of forefoot bursitis. These cases suggest that MSUS scanning of the plantar surface of the heel may be useful for patients with plantar heel pain who are suspected of having a very early phase of RA, because proliferative synovitis can be detected as subcalcaneal bursitis.