The cytologic diagnosis of a thymoma by transthoracic fine needle aspiration is described. A 64-year-old female was found toi have a left hilar mass following an episode of hemoptysis. Fine needle aspiration of the lesion demonstrated an intimate admixture of two cell types: epithelial and lymphoid. On the basis of the biphasic cytomorphology, the diagnosis of thymoma was suggested; this was confirmed by light and electron microscopic examination of tissue obtained at thoracotomy. Because of the paucity of cytologic descriptions of thymoma in the literature, pertinent morphologic features are emphasized, differential diagnostic possibilities are discussed, and the value of ultrastructural studies is confirmed.