We report the case of a 22-year-old man with a ACTH-producing primary thymic atypical carcinoid, in whom tumor cells were detected in the pericardial fluid. Pericardial fluid cytology performed in a specimen obtained during surgery revealed tumor cells in clusters, with round to ovoid nuclei and enlarged nucleoli, a high nuclear/cytoplasmic ratio, and fine to coarse granular chromatin. Immunocytochemically, the tumor cells were positive for chromogranin A and ACTH. Based on these findings, we made the diagnosis of carcinoid. The diagnosis of carcinoid can be suspected from the cytological features, and immunocytochemical staining for neuroendocrine markers can yield a definitive diagnosis. However, assertive differentiation between typical and atypical carcinoid is difficult by cytology alone.