Objective., To refine the value of baseline and adrenocorticotropin hormone (ACTH)-stimulated cortisol levels in relation to mortality from severe sepsis or septic shock. Design. Retrospective multicenter cohort study. Setting: Twenty European intensive care units. Patients. Patients included 477 patients with severe sepsis and septic shock who had undergone an ACTH stimulation test on the day of the onset of severe sepsis. Interventions: None. Measurements and Main Results: Compared with survivors, nonsurvivors had higher baseline cortisol levels (29.5 +/- 33.5 vs. 24.3 +/- 16.5 mu g/dL, p =.03) but similar peak cortisol values (37.6 +/- 40.2 vs. 35.2 +/- 22.9 mu g/dL, p =.42). Thus, nonsurvivors had lower Delta max (i.e., peak cortisol minus baseline cortisol) (6.4 +/- 22.6 vs. 10.9 +/- 12.9 mu g/dL, p =.006). Patients with either baseline cortisol levels