Pancoast syndrome is usually secondary to lung cancer. We report a patient with Pancoast syndrome in whom a biopsy specimen of a cervical mass at first thought to be anaplastic carcinoma was found to stain positively for leukocyte common antigen and negatively for keratin, epithelial membrane antigen, and alpha-fetoprotein. A diagnosis of malignant lymphoma associated with Pancoast syndrome was made, and a salutary response was observed after combination chemotherapy. To our knowledge, this is the first reported case of Pancoast syndrome associated with malignant lymphoma. We emphasize the importance of using immunohistochemical stains to define the pathologic condition in difficult cases.