Delirium in the intensive care unit (ICU) is a serious complication associated with a poor outcome in critically ill patients. In this prospective observational study of the effect of a delay in delirium therapy on mortality rate, 418 ICU patients were regularly assessed using the Delirium Detection Score (DDS). The departmental standard required that if delirium was diagnosed (DDS >7), therapy should be started within 24 h. In total, 204 patients (48.8%) were delirious during their ICU stay. In 184 of the delirious patients (90.2%), therapy was started within 24 h; in 20 patients (9.8%), therapy was delayed. During their ICU stay, patients whose delirium treatment was delayed were more frequently mechanically ventilated, had more nosocomial infections (including pneumonia) and had a higher mortality rate than patients whose treatment was not delayed. Thus, it would appear that a delay in initiating delirium therapy in ICU patients was associated with increased mortality.