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SUMMARY Cyclic Cushing's syndrome (CS) due to thymic carcinoid is a rare disorder. We report a case of cyclic CS due to ectopic adrenocorticotropic hormone (ACTH)-secreting atypical thymic carcinoid tumor and reviewed similar cases published in the literature. Our patient had hypercortisolemia lasting approximately one month, followed by normal cortisol secretion, with relapse one year later. Histopathology revealed an atypical ACTH-positive thymic carcinoid. Ectopic CS can be derived from atypical thymic carcinoids, which can be aggressive tumors with early relapse, suggesting that this type of tumor probably needs aggressive treatment.