Cetuximab is a monoclonal antibody directed against the epidermal growth factor receptor and is indicated in the treatment of squamous cell carcinoma of the head and neck. The population pharmacokinetics of cetuximab were characterized by nonlinear mixed effects modeling (NONMEM V) using a total of 912 concentrations from 143 patients with recurrent and/or metastatic squamous cell carcinoma of the head and neck enrolled in 2 phase I/II studies. Cetuximab pharmacokinetics were best described by a 2-compartment model with Michaelis-Menten-type saturable elimination. Population estimates (between-subject variability, percent coefficient of variation) of the pharmacokinetic parameters were Vmax 4.38 mg/h (15.4%), Km 74 μg/mL, central compartment volume V1 2.83 L (18.6%), peripheral compartment volume 2.43 L (56.4%), and intercompartmental clearance 0.103 L/h (97.2%). Ideal body weight and white blood cell count were identified as predictors of Vmax and total body weight as a predictor of V1. Clinical dose adjustments beyond the approved body surface area-based dosing of cetuximab may be warranted in patients with extreme deviations of their actual body weight from ideal body weight. Agreement between simulated and measured concentrations monitored for up to 43 weeks of therapy indicates that cetuximab pharmacokinetic parameters remained constant during prolonged therapy.