We studied clinical and laboratorial features of 73 patients with endogenous Cushing's syndrome, subdivided as follows: 46 (63%) with Cushing's disease (CD), 21 (28.7%) with an adrenal tumor and 6 (8.2%) with ectopic ACTH secretion (EAS). The rate of typical manifestations of hypercortisolism was similar regardless its etiology. In 100% of cases of Cushing's syndrome we observed serum cortisol levels greater than 1.8 microg/dL in low-dose dexamethasone (DMS) suppression tests, as well as elevation of serum or salivary midnight cortisol. However, urinary free cortisol was normal in 11.5% of patients. ACTH levels were suppressed in patients with adrenal tumors, normal or high in CD and always high in EAS. In the 8-mg overnight DMS suppression test, serum cortisol suppression > 50% was observed in 78.2% of cases of CD and in 33.3% of subjects with EAS, while an 80% suppression was only seen in CD. After stimulation with CRH or DDAVP an ACTH increase > 35% occurred in 81% of individuals with CD and 16.6% of those with EAS, while an ACTH increase > 50 achieved 100% specificity. Moreover, the combination of serum cortisol suppression > 50% and an ACTH increase > 35% in both tests only occurred in Cushing's disease. Pituitary magnetic resonance imaging identified 100% of macroadenomas and 59.4% of microadenomas in patients with CD. Among 10 patients that underwent bilateral inferior petrosal sinus sampling, a central-to-peripheral ACTH gradient > 3 after CRH or DDAVP had 90% sensitivity and 100% specificity for Cushing's disease.