OBJECTIVE:For patients in intensive care units, sepsis is a common and potentially deadly complication and prompt initiation of appropriate antimicrobial therapy improves prognosis. The objective of this trial was to determine whether a strategy of antimicrobial spectrum escalation, guided by daily measurements of the biomarker procalcitonin, could reduce the time to appropriate therapy, thus improving survival. DESIGN:Randomized controlled open-label trial. SETTING:Nine multidisciplinary intensive care units across Denmark. PATIENTS:A total of 1,200 critically ill patients were included after meeting the following eligibility requirementsexpected intensive care unit stay of ≥24 hrs, nonpregnant, judged to not be harmed by blood sampling, bilirubin <40 mg/dL, and triglycerides <1000 mg/dL (not suspensive). INTERVENTIONS:Patients were randomized either to the “standard-of-care-only arm,” receiving treatment according to the current international guidelines and blinded to procalcitonin levels, or to the “procalcitonin arm,” in which current guidelines were supplemented with a drug-escalation algorithm and intensified diagnostics based on daily procalcitonin measurements. MEASUREMENTS AND MAIN RESULTS:The primary end point was death from any cause at day 28; this occurred for 31.5% (190 of 604) patients in the procalcitonin arm and for 32.0% (191 of 596) patients in the standard-of-care-only arm (absolute risk reduction, 0.6%; 95% confidence interval CI −4.7% to 5.9%). Length of stay in the intensive care unit was increased by one day (p = .004) in the procalcitonin arm, the rate of mechanical ventilation per day in the intensive care unit increased 4.9% (95% CI, 3.0–6.7%), and the relative risk of days with estimated glomerular filtration rate <60 mL/min/1.73 m was 1.21 (95% CI, 1.15–1.27). CONCLUSIONS:Procalcitonin-guided antimicrobial escalation in the intensive care unit did not improve survival and did lead to organ-related harm and prolonged admission to the intensive care unit. The procalcitonin strategy like the one used in this trial cannot be recommended.