Fifteen cases of a unique lymphadenitis called subacute necrotizing lymphadenitis were studied electron-microscopically. The large lymphoreticular cells proliferating at cortical or paracortical areas of the lymph nodes mainly consisted of immunoblasts and histiocytoid cells, which were characterized by numerous intracytoplasmic myelinlike inclusions. Such histiocytoid cells seemed to be derived from the immunoblasts. Tubuloreticular structures, which had been often noticed within endothelial cells or lymphocytes of the patients with systemic lupus erythematosus (SLE) or SLE-related diseases, were also observed with high frequency in most cases examined. They were present within the cytoplasm of immunoblasts, endothelial cells, and histiocytoid cells. Immunoblasts in mitosis occasionally contained these structures. We offer the hypothesis that subacute necrotizing lymphadenitis with still unknown etiology may reflect a self-limited SLE-like autoimmune condition induced by virus-infected transformed lymphocytes.