A 21-year-old man with signs and symptoms of rapidly progressive shock was admitted to the intensive care unit for treatment of suspected sepsis. Levels of inflammatory markers (including procalcitonin) were highly elevated, but no obvious focus of infection was apparent. Initial sepsis therapy included administration of broad-spectrum antibiotics, vasoconstrictors, and drotrecogin alfa. Cultures of blood, sputum, and urine showed no growth, and no viruses were detected. The random (no stimulation with corticotropin) cortisol level at admission was less than 25 nmol/L. Assays for autoantibodies to the adrenal cortex were strongly positive and confirmed the diagnosis of adrenal failure caused by Addison disease. After initiation of steroid therapy, the patient fully recovered. Although increased procalcitonin levels are considered a reliable and specific indicator of severe generalized infections and bacterial sepsis, elevated procalcitonin levels cannot be relied on when trying to differentiate between addisonian crisis and septic shock.