Vasculitis is a syndrome which may complicate certain infectious, rheumatic, and allergic diseases. We identified 13 patients, over the past 17 years, who had both vasculitis and lympho- or myeloproliferative disorders and relate their clinical, laboratory, histologic, and immunologic features, course, therapy, and outcome. Nine patients were male, 4 female; ages ranged from 28 to 82 years. Ten of 13 patients presented with cutaneous vasculitis antedating malignancy by an average of 10 months. Three of 13 developed cutaneous vasculitis after malignancy. A statistically significant association between cutaneous vasculitis and lympho- or myeloproliferative malignancies was noted when compared with all other tumors. Dermatologic manifestations included palpable purpura (5 patients), maculopapular eruptions (4), urticarial and petechial lesions (3), and ulcers (1). Hepatitis B surface antigen, Coombs antibodies, rheumatoid factor and antinuclear antibodies were not found. Serum cryoglobulins were detected in 3 patients; serum C3 and C4 were normal in 8 of 9 patients evaluated. Histologic examinations revealed necrotizing leukocytoclastic vasculitis with disruption of endothelial integrity, destruction of endothelium, and neutrophil infiltration. Occasional perivascular mononuclear cell invasion was also noted in 4 patients. Immunofluorescent staining for IgG, IgA, IgM, C3, and C4 was negative in all patients studied. Symptoms were, in general, poorly responsive to therapy, which included nonsteroidal antiinflammatory drugs, antihistamines, antiserotonin agents, and corticosteroids. Chemotherapy directed at the underlying malignancy was also generally ineffective, although the vasculitis appeared to lessen in severity. Vasculitis appeared to lessen in severity as bone marrow function deteriorated. Ten patients died, all as a direct result of their malignancy. We have described a unique clinical syndrome of lympho- and myeloproliferative disease presenting with small-vessel vasculitis. Recognition that rheumatic symptoms may reflect or antedate malignancy may permit early diagnosis, aggressive treatment, and elucidation of pathogenesis.