Macrophage activation syndrome (MAS) is a life-threatening complication of rheumatic diseases that is thought to be caused by the activation and uncontrolled proliferation of T lymphocytes and macrophages, leading to widespread haemophagocytosis and cytokine overproduction. It is seen most commonly in systemic juvenile idiopathic arthritis, but is increasingly recognized also in juvenile systemic lupus erythematosus (J-SLE). Recognition of MAS in patients with J-SLE is often challenging because it may mimic the clinical features of the underlying disease or be confused with an infectious complication. This review summarizes the characteristics of patients with J-SLEassociated MAS reported in the literature or seen by the authors and analyses the distinctive clinical, diagnostic and therapeutic issues that the occurrence of MAS may raise in patients with J-SLE. Lupus (2007) 16, 587—592.