Histological appearances of the spleen and lymph nodes were analysed in twenty fatal cases of human visceral leishmaniasis from Sudan. Marked atrophy of the splenic white pulp was associated with necrosis and fibrosis of thymus-dependent area, accumulation of parasite-containing histiocytes and plasma cell hyperplasia. Depletion of small lymphocytes in the paracortical areas of the lymph nodes was accomplained by proliferation of plasma cells and histiocytes in the paracortex. Depletion of small lymphocytes in thymus-dependent regions of lymph nodes and spleen is viewed as arising from immune suppression associated with antigen overloading or other factors, which may impair those aspects of lymphocyte-macrophage cooperation that are presumably necessary to kill the invading parasites.