Unilateral tonsillar enlargement may result from infection, chronic inflammatory response, or neoplasm. Neoplasms that commonly produce a unilaterally enlarged tonsil include lymphomas (lymphocytic and histiocytic types) and squamous cell carcinomas. Rarer tumors include extramedullary plasmacytomas, Hodgkin's disease, leukemia, and metastatic neoplasms. Sixteen cases of unilateral tonsillar enlargement owing to causes other than squamous cell carcinoma are reviewed. When examining a patient with unilateral tonsillar enlargement, diagnosis of a neoplastic disease must be considered.