The clinical and pathologic findings of four patients with Chediak-Higashi syndrome in the accelerated phase were studied in order to clarify the nature of this enigmatic process. Fever, lymphadenopathy, hepatosplenomegaly, and cytopenias were present in every patient. All cases demonstrated extensive parenchymal infiltrates in many organs composed of benign-appearing histiocytes manifesting hemophagocytosis accompanied by lymphocytes and plasma cells. Studies in one patient suggested a viral etiology with the findings of a low blood lymphocyte OKT4 to OKT8 ratio, acquired loss of lymphocyte response to mitogens, the presence of Epstein-Barr virus genome in the mononuclear cells of lymph node, blood, and bone marrow, and possible clinical responses to acyclovir. It is concluded that the accelerated phase of Chediak-Higashi syndrome may be the clinicopathologic expression of the virus-associated hemophagocytic syndrome.