In the last 3 decades, there have been several advances in understanding the pathogenesis of Cushing's syndrome and in testing for the diagnosis and differential diagnosis of its various forms. Advanced diagnostic techniques provide useful tools in discovering ectopic adrenocorticotropic hormone sources. However, the occurrence of unusual clinical presentations, laboratory shortcomings, and exogenous compound interference may lead to wrong conclusions. This article reviews the atypical presentations of hypercortisolism and some laboratory shortcomings that may confuse the diagnosis of Cushing's syndrome. Comments and suggestions are given with the aim of helping the clinician avoid diagnostic mistakes.