CD105 (endoglin) is expressed significantly in activated endothelial cells in culture and in tumor microvessels. Quantification of CD105 immunocytochemical expression that may be clinically relevant has not been accurately evaluated. We studied CD105 expression on frozen tissue sections by using immunohistochemical assays in a series of 929 patients and correlated the findings with long-term follow-up (median, 11.3 years). Univariate (Kaplan-Meier) analysis showed that the number of CD105+ microvessels (cutoff, 15 vessels) correlated significantly with poor overall survival among all patients (P = .001). This correlation was less significant in node-negative patients (P = .035). Marked CD105 expression also correlated with a high risk for metastasis among all patients (P = .006) and among node-negative patients (P = .001). Multivariate analysis (Cox model) identified CD105 immunodetection as an independent prognostic indicator. Our results suggest that immunohistochemical expression of CD105 has practical clinical relevance for identifying node-negative patients with a poor prognosis. Moreover, immunodetection of CD105 also may be considered a potential tool for selecting patients who could benefit from specific antiangiogenic therapy, using anti-CD105 conjugates.