Familial hemophagocytic lymphohistiocytosis (FHL), which typically has its onset during infancy, is uniformly fatal if not treated. It therefore requires prompt therapeutic intervention. Although hyperferritinemia has been emphasized as a useful marker for FHL, some nonfatal cases in infants with spontaneous remission also manifest with hyperferritinemia. However, distinguishing them is difficult because initial clinical features of these infants are similar. The authors encountered 14 infants with hyperferritinemia (serum ferritin >674 ng/mL), which normalized within 3 weeks following a benign clinical course. The authors compared the levels of HLA-DR+CD3+ T-cell subsets and interferon-gamma (IFN-γ) in the peripheral blood between these infants and FHL cases: one of the authors' own patients and others from the literature. Serum IFN-γ was not detected in infants with hyperferritinemia. Moreover, levels of HLA-DR+CD3+ T cells were extremely depressed. In contrast, serum IFN-γ was elevated and HLA-DR+CD3+ T cells were not depressed in FHL. Measurement of activated T cells and serum IFN-γ might help differentiate FHL in febrile infants with transient hyperferritinemia.