We describe two 3-year-old patients with systemic juvenile idiopathic arthritis (SJIA) who developed hepatitis A-associated macrophage activation syndrome (MAS). One patient showed MAS as the presenting manifestation of SJIA, while MAS complicated SJIA during the second year of the disease course in the other child. Both girls presented with fever, jaundice, hepatosplenomegaly, neurological involvement, mucosal hemorrhage, and purpura. Cytopenias, hypofibrinogenemia, and hemophagocytosis confirmed the diagnosis. After aggressive treatment with high-dose corticosteroids and immunosuppressants one patient entered remission while the other one died. Hepatitis A virus may induce severe MAS in SJIA.