The clinico‐pathological, immunohistochemical and ultrastructural features of eight cases of sinus histiocytosis with massive lymphadenopathy (SHML) recorded in Rwanda from 1975 to 1980 are reported.The main histopathological features were a massive enlargement of the lymph node sinuses which were filled with large histiocytes, a great number of which phagocytosed blood cells, especially lymphocytes, and an important proliferation of plasma cells in the medullary cords. The pathogenesis of this syndrome is discussed, particularly the role of immunological disorders challenged by a chronic, but non‐specific, infectious state. The ultrastructural study revealed no micro‐organisms and no Langerhans’ granules. Immunohistochemical staining revealed two interesting features: the presence of immunoglobulins in the cytoplasm of the histiocytes indicating the presence of an immune phagocytosis and the absence of lysozyme and α1‐antichymotrypsin (α1‐AC) from the histiocytes. The significance of the latter phenomenon is