OBJECTIVE Cytopathologic descriptions of thymoma are limited. This study was undertaken to elaborate on the cytomorphologic features along with clinicoradiologic findings of this entity, with special emphasis on the differential diagnosis. METHODS Twenty cases of thymoma were retrospectively studied. Material was obtained by fine needle aspiration (FNA). Smears were stained with Diff-Quik and Papanicolaou stain, and cell block sections were stained with hematoxylin and eosin. Immunocytochemical (ICC) studies were done in selected cases using cytokeratins, epithelial membrane antigen, carcinoembryonic antigen, placental alkaline phosphatase, neuron-specific enolase, chromogranin and lymphocytic markers (CD3 and UCHL-1). RESULTS Fourteen thymomas were from the anterior mediastinum; the remaining 6 were invasive or metastatic to the lung (2), pleura (2), liver (1) and abdominal wall (1). The key diagnostic feature was a biphasic population of epithelial cells and lymphocytes in varying proportions. Medullary thymomas showed cohesive, spindled epithelial cells, whereas cortical subtypes displayed more epithelioid epithelial cells singly and in discohesive clusters. All cases of invasive or metastatic thymoma were of the cortical subtype. ICC studies were positive for cytokeratins and epithelial membrane antigen. CONCLUSIONS FNA, coupled with clinicoradiologic information and appropriate immunocytochemical studies, is highly accurate in the diagnosis of primary and metastatic thymomas.