CONTEXT: Discriminating Cushing disease (CD) from pseudo-Cushing syndrome (PCS) is a challenging task that may be overcome with the 4-mg intravenous (IV) dexamethasone suppression test (DST). OBJECTIVE: Assess the performance of the 4-mg IV DST in the differential diagnosis between CD and PCS in well-characterized patients. DESIGN: Retrospective comparative study of subjects seen in a tertiary care unit (November 2008 to July 2011). METHODS: Thirty-six patients with PCS and 32 patients with CD underwent 4-mg IV dexamethasone infusions from 11 am to 3 pm. Areas Under ROC Curves (AUCs) were estimated and compared for ACTH and cortisol measured at 4 pm the same day (day 1) and 8 am the next day (day 2). The ROC curve of the marker with the highest AUC was used to determine the threshold with the highest specificity for 100% sensitivity. RESULTS: The AUC of ACTH at 8 am on day 2 was estimated at 98.4% (95% CI: [92.1-100]), which is significantly greater than that of ACTH at 4 pm on day 1 (P=0.04) and that of cortisol at 8 am on day 2 (P=0.05). For ACTH at 8 am on day 2, the threshold with the highest specificity for 100% sensitivity was estimated at 14.8ng/L. At this threshold, the sensitivity was estimated at 100% [89-100] and the specificity at 83.3% [67-94]. CONCLUSION: The 4-mg IV DST is an easy and accurate tool in distinguishing CD from PCS. It deserves thus a better place in establishing the diagnosis of CD.