In a patient with hairy cell leukemia, segmental vascular alterations developed in many of the larger visceral arteries. In previously reported instances of this association, the vasculitis resembled polyarteritis nodosa. In contrast, in the present case the vessels did not show the typical necroses, polymorphonuclear infiltrates, and aneurysmal changes. Instead, vascular alterations consisted of edema, fibrosis, and infiltration by tumor cells. A possible pathogenetic mechanism for these findings may be a local immunologic interaction of tumor cells and vessels, suggesting a direct relation between hairy cell leukemia and vascular damage.