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Objectives To characterise the clinical presentation, laboratory findings, treatment approaches, and outcomes of systemic lupus erythematosus (SLE)-associated macrophage activation syndrome (MAS), and compare these features with other systemic rheumatic disease (SRD)-related cases within a Portuguese multicentric cohort.Methods This nationwide retrospective study included adult patients with MAS followed in 10 Portuguese rheumatology centers until July 2025. MAS was defined by fulfilment of at least one of the following criteria: hemophagocytic lymphohistiocytosis (HLH)2004, HScore, or PRINTO criteria. Patients with primary or non-rheumatic HLH were excluded. Demographic, clinical, laboratory, therapeutic, and outcome data were collected and compared across subgroups.Results 42 MAS episodes were identified, 19 SLE-associated. These occurred predominantly in women (78.9%), with a median age of 36.9 years (IQR 26.2–50.6). The median age at SLE diagnosis was 26.8 years (IQR 20.4–26.7), indicating that MAS typically developed several years after disease onset. Most episodes arose during active lupus flare (84.2%), with infection acting as a concomitant trigger in 42.1%. Fever (100%), cytopenias (73.7%), hepatomegaly (64.7%), and lymphadenopathy (63.2%) were common findings. Median ferritin was 5160 ng/mL (IQR 3120–7820), and median HScore 216 points (IQR 193–226), exceeding the diagnostic threshold in 89.5%. Ear, nose, and throat (ENT) involvement was more frequent among non-SLE MAS (26.1% vs 0%; p = 0.024). Organ dysfunction occurred in 68.4% (13/19), mainly haematological (63.2%) and neurological (31.6%). ICU admission was required in 31.6% (6/19), and in-hospital mortality reached 33.3% (6/18). Neurological involvement was significantly associated with the need for intensive care and organ support (OR 11.0; 95% CI 1.0–119.9; p = 0.047), mainly reflecting mechanical ventilation and vasopressor requirement, and showed a trend toward increased mortality (OR 10.0; 95% CI 1.03–97.5; p = 0.107). Severe thrombocytopenia was also associated with ICU admission (median 42 × 10^9/L vs 132 × 10^9/L; p = 0.020). Neither HScore nor ferritin > 6000 ng/mL differed between survivors and non-survivors, and the presence of an infectious trigger was not associated with increased organ dysfunction or mortality.Abstract PT3:08 Table 1Baseline demographic, clinical, and laboratory features of MAS according to underlying systemic rheumatic disease (SLE vs non-SLE)Conclusions SLE-associated MAS remains a severe and heterogeneous complication with frequent multiorgan dysfunction and substantial mortality. Neurological involvement and thrombocytopenia emerged as potential early prognostic indicators warranting prompt therapeutic escalation.