The objective of this study was to assess the anatomical variation of the marginal mandibular nerve, and evaluate the risk of nerve malfunction after neck dissection. The method involved clinical assessment of the anatomy and function of the marginal mandibular nerve in 133 neck dissections. When the neck was extended the nerve was displaced in an anterior and downward direction with the lowest point 1.25+/-0.7 cm below the mandible between the posterior and anterior facial veins. The nerve was >1cm below the lower border of the mandible in 54% of dissections. When the intent was to preserve the nerve, dysfunction was observed in 16 of 101 dissections (16%). The incidence of marginal mandibular nerve dysfunction following neck dissection is comparable to that observed following submandibular gland excision for benign disease. Placement of incisions 2 cm below the lower border of the mandible will put the nerve at risk in a significant number of patients.