Macrophage activation syndrome (MAS) is a rare and potentially fatal disorder, thought to result from uncontrolled activation and proliferation of T cells and excessive activation of macrophages. The term MAS designates a clinicopathologic entity that occurs in different hemophagocytic syndromes (HSs). Primary hemophagocytic lymphohistiocytosis (HLH) is recognized to have an immunogenetic basis, but in the secondary HS (also referred to as secondary HLH), the cause is unknown. The pathogenesis of the accelerated disease phase typical of MAS remains incompletely understood. This report describes the immunohistochemical findings on liver tissues from 5 children, each of whom presented with MAS in the context of a different type of HS. The data provide direct evidence for the involvement of activated CD8(+) lymphocytes through the production of interferon-gamma and of macrophages through hemophagocytosis and production of interleukin 6 and tumor necrosis factor-alpha, and underscore the view that MAS in different HSs share a common effector pathway.