Purpose: To assess the pharmacodynamic effects of nimotuzumab, an anti-epidermal growth factor receptor (EGFR) monoclonal antibody with intermediate affinity for the receptor, in skin and tumor tissues from head and neck cancer patients. Experimental design: Pharmacodynamic study in patients with advanced squamous cell carcinoma of the head and neck, unsuitable for chemoradiotherapy, enrolled in a single-center trial. Patients received 8 weekly infusions of nimotuzumab. The first nimotuzumab infusion was administered 1 week before starting radiation, whereas the remaining doses were administered concomitantly with irradiation. Paired biopsies were taken from skin and primary tumors, before (pretherapy) and 1 week (on single-agent therapy) after first infusion. Immunohistochemistry was conducted to assay the effects of nimotuzumab on total and phosphorylated EGFR, phosphorylated extracellular signal-regulated kinase 1/2 (p-ERK1/2), p-AKT, and proliferation (Ki-67). Results: Nimotuzumab was well tolerated and there was no evidence of skin rash. Objective response was achieved in 9 of 10 patients. The pharmacodynamic assays showed inhibition of p-EGFR in both skin and tumor (P = 0.042 in skin and P = 0.034 in tumor). No significant changes in p-ERK1/2, p-AKT, or Ki-67 were detected in skin. In addition, lymphocytic infiltrates, folliculitis, or perifolliculitis were not observed. In tumor samples, there was an upregulation of p-AKT (P = 0.043), a reduction in proliferation index (P = 0.012), and a nonsignificant trend toward a decrease of p-ERK1/2 (P = 0.091). Conclusions: The pharmacodynamic data confirmed the ability of nimotuzumab to decrease EGFR phosphorylation. Downstream effects were observed in tumor cells but not in skin, a finding that may help to explain the lack of skin rash in patients treated with nimotuzumab. ©2010 AACR.