Cetuximab is a monoclonal antibody targeting the epidermal growth factor receptor. We encountered two cases of hypopharyngeal carcinoma that developed interstitial pneumonia caused by the combination of cetuximab and radiotherapy. Case 1: A 78-year-old male with organizing pneumonia-type lung injury was treated with prednisolone and improved. Case 2: A 77-year-old male with diffuse alveolar damage- type lung injury was treated with methylprednisolone pulse therapy and respirator management. However, after an improvement in the pulmonary lesions, he developed bacterial pneumonia and a pneumothorax, and died. Both cases demonstrated high fever, increased inflammatory response, and elevated serum levels of surfactant protein D. It is suggested that the incidence of interstitial pneumonia related to cetuximab for head and neck cancer is higher than that for colorectal cancer and the onset of interstitial pneumonia due to cetuximab may be related to its being administered in combination with radiotherapy.