PURPOSE: To determine whether Cy5.5-labeled antiepidermal growth factor (EGFR) antibody could be used to detect head and neck squamous cell carcinoma (HNSCC) xenografts in vivo. METHODS: AntiEGFR antibody (cetuximab) was labeled with Cy5.5, a fluorophore with emission in the near infrared range. The cetuximab-Cy5.5 conjugate was systemically administered in subtherapeutic doses (50 μg) to mice bearing orthotopically xenografted HNSCC cell lines (SCC1, CAL27, and FaDu). As a control, isotype-matched human immunoglobulin (Ig)G1k antibody labeled with Cy5.5 was systemically injected in parallel experiments. All tumor regions (n = 6) were imaged by fluorescent stereomicroscopy at 0, 6, 24, 48, or 72 hours. Tumor size was measured by high-frequency ultrasonography at 72 hours. Transcervical partial and near-total resections were then performed with stereomicroscopic imaging after each resection. The mandible and associated structures were then resected, paraffin embedded, and then serial sectioned for analysis. RESULTS: Tumors could be clearly visualized by near infrared fluorescent stereomicroscopy at 48 and 72 hours after systemic administration of cetuximab-Cy5.5 but not after administration with the labeled isotype control antibody, IgG1k-Cy5.5. Ultrasound measurement of tumors (n = 5) correlated with fluorescent measurements of tumor (Spearman's coefficient, 0.92, P ≤ .01). When fluorescent stereomicroscopic findings were correlated with histologic findings in near-total resections, this technique could accurately identify residual tumor less than 1 mm in size. CONCLUSION: Fluorescent immunoguided neoplasm detection may be used as a diagnostic tool and to guide surgical therapy by providing real-time imaging information about the extent of disease or the presence of residual disease. © 2006 The American Laryngological, Rhinological and Otological Society, Inc.