Merkel cell carcinomas are aggressive tumours for which histological prognostic factors need to be established. This study examines the prognostic role of vascular density, based on CD34 immunohistochemical staining in Merkel cell carcinoma. Thirty-six cases of Merkel cell carcinoma were immunohistochemically stained for the endothelial marker CD34. Vascular density was assessed in the tumor and stroma with a Chalkley eyepiece graticule. The scores of vascular density were correlated with other clinical and histological parameters to determine the prognostic significance of tumor vascularity. Increased vascular density was shown to be significantly associated with a worse prognosis (P = 0.005). A 1-unit increase in total vessel score was associated with a 3.9 times increase in the risk of death (95% hazard ratio confidence limits 1.50-10.32). Other factors associated with a worse outcome included tumor size (P = 0.05), the presence of lymphovascular invasion (P = 0.03), and tumor mast cell count (P < 0.002). Increased vascular density is associated with a worse prognosis in Merkel cell carcinomas. Assessment of vascular density may assist in predicting clinical behavior in these tumors and in evaluating the effects of adjuvant therapy.