A variety of malignant neoplasms have been shown to induce capillary neovascularization, and in some cases the degree of vascularization appears to correlate with aggressive behavior and risk of metastasis. We hypothesized that carcinoma of the prostate also induces the formation of new capillaries, and we developed a method to quantify the relative density of microscopic vessels in carcinoma of the prostate compared with benign prostatic glandular tissue. The number of microvessel profiles in tissue sections was quantified by marking the vascular endothelial cells with antibodies to factor VIII-related antigen using standard immunohistochemistry techniques and comparing fields of adenocarcinoma with benign glandular tissue in 15 radical prostatectomy specimens. The analysis was facilitated by using the Optimas computerized image analysis system (Bioscan, Seattle, WA) with software written for this investigation. Fourteen of the 15 cases demonstrated significantly higher vascular density in the areas of carcinoma than in the benign tissues. Overall, the ratio of vessels per unit area in sections of carcinoma versus benign tissue was approximately double (ratio = 2.02; P < .001). In benign tissues the capillaries are restricted for the most part to the periglandular stroma immediately adjacent to the epithelium, whereas the distribution in carcinoma appears to be more random. The data demonstrate the increased density of capillaries in prostatic carcinoma when compared with benign prostate tissue.