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Objectives Systemic lupus erythematosus (SLE) and systemic sclerosis (SSc) are distinct connective tissue diseases with unique clinical and immunological features, both characterized by multisystem involvement. Overlap syndromes occur when features meet criteria for more than one disease, but a phenotype switch from SSc to SLE has been rarely described.Methods We report a rare case of a clinical phenotype transformation from SSc to SLE.Results A 41-year-old Afro-Caribbean woman was diagnosed in 2015 with anti-Scl-70 positive diffuse cutaneous SSc with polymyositis overlap. Initial features included Raynaud’s phenomenon, digital ulcers, progressive skin thickening, microstomia and proximal muscle weakness. Laboratory evaluation showed ANA 1:5120 (homogenous/nucleolar pattern), anti-Scl-70 and anti-Ku positivity and normal anti-dsDNA and complement levels. Over time, she developed severe hand contractures, acro-osteolysis ( figure 1), interstitial lung disease and gastrointestinal involvement. She was treated with mycophenolate mofetil for skin and lung disease, prednisolone for myositis, and maintained on losartan and iloprost for vasculopathy. In January 2025, she presented with 12 weeks of fatigue, proximal myalgia, pleuritic chest pain, dyspnoea, peripheral oedema and 14% weight loss. Examination showed mild weakness and small cervical lymphadenopathy. Tests revealed bicytopenia, elevated inflammatory markers, moderate proteinuria, preserved renal function, polyclonal hypergammaglobulinemia, elevated NT-proBNP, normal troponins and negative viral serologies. Repeat autoantibody testing showed persistently high ANA (1:5120, homogeneous pattern), new anti-dsDNA elevation and hypocomplementemia (figure 2). Imaging revealed moderate pericardial effusion, lymphadenopathy, and chronic SSc-related myocardial damage, without evidence of myocarditis or myositis. These findings suggested a phenotype shift from SSc to SLE. She was treated with intravenous methylprednisolone pulses, oral prednisolone, intravenous cyclophosphamide (EURO-Lupus protocol) and heart failure therapy. Clinical and laboratory improvement followed, with recovery of hematologic parameters, complement levels, reduction in anti-dsDNA (figure 2), inflammatory markers and proteinuria, and resolution of pericardial effusion and lymphadenopathy.Abstract PO:04:122 Figure 1–2Conclusions A clinical phenotype switch from SSc to SLE is extremely rare, with only one similar case reported. The underlying mechanisms are unclear but may reflect immune plasticity in the setting of a chronic inflammatory milieu. Clinicians should therefore reconsider diagnoses when unexpected features arise. Early and thorough reassessment, including serological re-testing, is essential to ensure timely recognition and management.