Serious renal involvement in systemic diseases is common and generally constitutes a pivotal prognostic factor, making those pathology frequently seen in nephrology departments. Authors describe the case of a nine-year-old girl with lupus nephritis. After admission the patient's state deteriorated over a period of a few days, with an unremitting high-grade fever, significant weakness and drowsiness, generalized erythema, and decrease of the kidney function to eGFR nadir of 56 ml/min/1,73m 2 . Treatment with pulsed methylprednisolone was started. After the first pulse the general state of the patient improved slightly, although laboratory tests showed an alarming evolution, with the exacerbation of anemia, leukopenia, neutropenia, increase of serum CRP concentration, extremely high D-dimer concentration and increase in activity of lactate dehydrogenase. The concentration of ferritin rose reaching the level of 540 μg/l, triglicerydes level was also high. Intravenous cyclophosphamide pulse therapy was added to the ongoing steroid treatment, and resulted in a radical patient improvement. Authors underline that it seems important to be aware of rare, non-renal, but potentially devastating complications of systemic diseases, like in this clinical case: the secondary hemophagocytic lymphohistiocytosis (HLH). When HLH complicates a rheumatic disease, it is also referred to as macrophage activation syndrome (MAS). Unfortunately treatment of MAS is still based on reports provided by individual centres and gathered own experiences so drawing up unambiguous diagnostic criteria will be valuable in future. The treatment should be individually tailored, and more specific evidence-based recommendations are needed.