Forty-two cases of vasculitis coincident with hairy cell leukemia (HCL) have been reported, of which 17 had panarteritis nodosa (PAN), 21 had cutaneous leukocytoclastic vasculitis (LCV), and 4 had vessel wall infiltration by hairy cells. PAN generally occurred after the diagnosis of HCL, splenectomy, and infection. HBs antigen was detected in 3 of 12 patients tested, whereas immune complexes were positive in 3 of 4 patients tested. LCV was often preceded by infection and was frequently detected before HCL. Serum immunoglobulin levels were generally elevated when measured. Cryoglobulins, complement, rheumatoid factor, and antinuclear antibodies showed no clear association with vasculitis in HCL. These reports suggest a role for infection and splenectomy as contributing factors to vasculitis.