A case of sinus histiocytosis with massive lymphadenopathy in a boy of 13 with multiple subcutaneous tumor-like formations 0.5 to 2.5 cm in diameter is described. The general condition of the patient was not changed despite the 7-month course of the disease. Mild anemia and increased ESR were observed. Microscopically the formations presented massive "dense" infiltrates of lymphoid, plasma cells, and histiocytes-macrophages with foci of fibrosis and xanthomatosis. Since the morphological picture of sinus histiocytosis with massive lymphadenopathy may to some extent imitate the substrate of some tumor diseases of the hemopoietic and lymphoid tissues (malignant histiocytosis, histiocytosis X, lymphogranulomatosis, lymphosarcoma), their differential diagnosis is presented.