“Successful” adrenal vein catheterization in primary aldosteronism (PA) is often defined by a ratio of >3:1 of cortisol in the adrenal vein vs the inferior vena cava. Non-use of corticotropin (ACTH) during sampling may increase the apparent failure rate of adrenal vein catheterization due to lower cortisol levels. A retrospective study was performed on all patients with confirmed unilateral PA between June 2005 and August 2011. Adrenal vein sampling (AVS) included simultaneous bilateral baseline samples with repeat sampling 15 minutes after intravenous infusion of 250 μg of Cortrosyn (ACTH-S). Successful catheter placement was judged as adrenal cortisol:IVC cortisol of >3:1, applied to both baseline and ACTH-S samples and lateralization of aldosteronism was judged as normalized aldosterone/cortisol (A/C) ratio >3 times the contralateral A/C ratio. In ACTH-S samples, 94% of right-sided catheterizations were biochemically successful with 100% success on the left. Among baseline samples, only 47% of right- and 44% of left-sided samples met the 3:1 cortisol criteria. However, 95% of apparent “failed” baseline cortisol sets still showed lateralization of A/C ratios that matched the ultimate pathology. Non–ACTH-stimulated samples may be incorrectly judged as failed catheter placement when a 3:1 ratio is used. ACTH-stimulated sampling is the preferred means to confirm catheterization during AVS.