Fifteen spleens with hairy cell leukemia are studied with immunofluorescent (7 cases), electron microscopic (5 cases), and usual histologic methods. The findings are: enlargment of the spleen (weight always superior to 400 g), diffuse red pulp infiltration by ambiguous cells with regular repartition of nuclei and clear spaces between them, hairy aspects of the cytoplasmic membrane which are especially observed on semi-thin and ultra-thin sections, presence of particular cytoplasmic inclusion bodies (polysome lamellae complex). The cellular infiltration is accompanied by some vascular modifications: pseudo angiomatosis lesions, nodular formations ressembling splenomas. An important hypertrophy of the splenic macrophages with erythrophagocytosis and siderosis is also observed. These two phenomena partly explain the anemia.