Prolonged exposition to supraphysiological doses of exogenous glucocorticoid eventually results in iatrogenic Cushing's syndrome, whose intensity depends on the dose and duration of the treatment and on individual susceptibility. In patients with chronic inflammatory diseases treated with oral glucocorticoids iatrogenic Cushing's is expected and recognized and it only imposes that the dose of glucocorticoid be maintained as low as possible and that there is no better alternative therapy available.In some cases, however, iatrogenic Cushing's syndrome may be unexpected by the prescribing physician as the true exposure to corticoids may depend largely on the patient: this is the case for topical steroids used in inflammatory skin diseases such as psoriasis. Factitious Cushing's syndrome (FCS) is another cause of exogenous Cushing's syndrome in whom the exposure to glucocorticoid is unexpected, as it is hidden to the physician by a patient suffering from Münchausen syndrome. FCS might be very difficult to diagnose depending on the type of glucocorticoid used, the specificity of the dosage used for cortisol, and the timing of the measurement of cortisol and ACTH. The best evidence for FCS is the demonstration by LC-MS/MS of exogenous glucocorticoid in his urine or plasma but this requires that the patient has not stopped to take glucocorticoid at the time of exploration. FCS related to hydrocortisone can be difficult to prove and to distinguish from cyclical Cushing's syndrome. Analysis of the literature shows that FCS has led to prolonged or invasive explorations and even to adrenal surgery, while unrecognized FCS has led to fatal infectious complications.