OBJECTIVE: To retrospectively analyze the differences in musculoskeletal ultrasound (MSUS) findings to distinguish patients with remitting seronegative symmetrical synovitis with pitting edema (RS3PE) syndrome and patients with elderly-onset rheumatoid arthritis (EORA). METHODS: We consecutively recruited patients with RS3PE syndrome (n = 7) and EORA (n = 22) who underwent pre-treatment MSUS of both hands. Synovial hypertrophy and vascularity of articular synovitis and those of tenosynovitis of the digital flexor tendons and the carpal extensor tendon were evaluated by gray-scale and power Doppler, respectively on a semi-quantitative scale (0-3). The presence/absence of intra-articular synovial effusion, bone erosion, peritendinitis of the digital extensor tendon, and subcutaneous edema were noted. RESULTS: Compared to the EORA group, mild articular synovitis was observed more extensively, and the frequency of intra-articular synovial effusion was significantly higher in the RS3PE syndrome group. Severe articular synovial hypertrophy was more frequent in the EORA group compared to the RS3PE syndrome group, and bone erosion was observed in some EORA cases. Tenosynovitis of the digital flexor tendon was more frequent and severe in the RS3PE syndrome group compared to the EORA group. Although the frequency and severity of tenosynovitis of the carpal extensor tendon were similar in the two groups, digital extensor tendon peritendinitis was more frequent in the RS3PE syndrome group. CONCLUSION: To distinguish patients with RS3PE syndrome from those with EORA, it is important to evaluate not only intra-articular lesions but also extra-articular lesions by MSUS.