Histopathologic features of 5 cases (4 boys and 1 girl; 4-9 years old) with severe chronic active Epstein-Barr virus (EBV) infection are discussed. All patients died within 3 years after disease onset without developing hematolymphoid malignant neoplasms. The pathology specimens (autopsy, 2 cases; multiple organs and tissues obtained by surgery or biopsy, 3 cases) showed polymorphic lymphocytic proliferation in the lymph nodes (4/5) and spleen (3/3), and systemic lymphocytic infiltration of the liver (4/4), lung (2/2), bone marrow (3/4), and kidney (2/2). Skin lesions were noted clinically in 3 of 5 cases. Two cases had coronary artery aneurysm due to lymphocytic vasculitis. The lymphocytes had a characteristic phenotype of cytotoxic T cells expressing CD3, CD8, and cytotoxic molecules, and were negative for CD4. EBV-encoded small nonpolyadenylated RNAs were detected in the nuclei of the lymphocytes, but latent membrane protein 1 and EBNA2 were not seen. In 4 of 4 cases, an oligoclonal growth pattern of EBV was determined after detecting terminal repetitive sequences by Southern blot. In 3 of 3 cases, the lymphocytes did not have T-cell receptor beta or J(H) gene rearrangement.