The morphological changes in 46 cases of primary paranasal aspergillus granuloma seen in the Sudan are described. Histologically, three groups can be differentiated: 1) a proliferative type, characterized by the presence of pseudotubercles in a fibrous tissue stroma; 2) an exudative-necrotizing type, consisting of large foci of edematous necrosis; and 3) a mixed group, where proliferation and necrosis exist side by side. Fungal hyphae are seen both within the giant cells and lying free in the stroma. Three factors seem to play a part in the pathogenesis of this disease: a) the climatic conditions, inducing recurrent inflammation of the nasal sinuses; b) the ability of the fungus and its toxic metabolites to cause damage; and c) an immunological process produced by the fungal antigens. The latter is supported by the finding of fibrinoid necrosis of collagen, vascular alterations, and formation of pseudotubercles.