Bone marrow samples from 30 patients with myelodysplastic syndromes (MDS) grouped according to the International Prognostic Scoring System for MDS were investigated for counts of microvessels, total metachromatic mast cells (MC) and MC expressing tryptase, an angiogenesis-inducing molecule. Counts were higher in patients with a poor prognosis. The observation of a high correlation between microvessel counts and both total metachromatic and tryptase-reactive MC in all samples suggests that angiogenesis in MDS increases with their progression and that MC may intervene in the angiogenic response in MDS through tryptase contained in their secretory granules.