We report 12 cases of leucocytoclastic vasculities associated with myelocytic (7 cases) or lymphocytic (5 cases) blood diseases. The clinical features, laboratory abnormalities and pathological findings are presented. In all cases a drug-induced and/or infective origin could be ruled out, and patients with cryoglobulinaemia were excluded. As in other cases found in the literature, in some patients vasculitis was present several weeks or months before the blood disease was discovered, while in others both conditions developed simultaneously or vasculitis appeared in the course of the blood disease. None of these three possibilities seemed to make the prognosis worse. The pathogenesis of the vasculitis-malignant blood disease association is uncertain. Immune complexes of tumoral origin undoubtedly have some responsibility, but some deficiency of phagocytic cells or chemotactic factors also play a role. Cases such as ours enable the significance of vasculitis to be understood when it occurs in a patient with malignant blood disease, but above all they should prompt investigations for malignant blood disease in patients with cutaneous and/or systemic vasculitis, as is being done in patients with pyoderma gangrenosum or Sweet's syndrome.