OBJECTIVES: To assess whether microvessel density (MVD), an immunohistochemical marker for tumor vascularity, predicts for radiotherapy (RT) outcome in locally advanced HNSCC patients. METHODS: A total of 459 patients, enrolled on the RTOG 90-03 trial, had biopsy specimens submitted, and a value for MVD determined, prior to definitive RT. 450 patients were analyzable for this study. Tumor microvessels were stained for factor VIII-related antigen using a standard immunoperoxidase method. The mean number of stained microvessel profiles, from three ×200 fields containing the highest MVD (hot spot), was recorded as the MVD. A prospective value of ≥60 was chosen as the threshold for high MVD, tumor vascularity. RESULTS: The median follow-up for the analyzable patients with MVD assessment was 22.0 months and 79.1 months for all living patients. There were no differences concerning the pretreatment characteristics between those RTOG 90-03 patients with a value for MVD and those without a value for MVD. Thus, the present study cohort possessed comparable characteristics with the entire RTOG 90-03 population. MVD values ranged from 5 to 80, with a median value of 30. Only 37 of 450 (8.2%) patients possessed an MVD ≥60. There were no outcome differences for patients with MVD <60 versus ≥60 on multivariate analysis for time to local-regional failure (P = 0.89), time to distant metastasis (P = 0.80), disease-free survival (P = 0.46), and overall survival (P = 0.39). CONCLUSIONS: In this large, correlative study, a MVD ≥60, ie, high tumor vascularity, did not predict for outcome in locally advanced head and neck squamous cell carcinoma patients treated with radiotherapy. © 2007 Lippincott Williams & Wilkins, Inc.