The Withdrawal Assessment Tool-1 (WAT-1) has been validated for assessing iatrogenic withdrawal syndrome in critically ill children receiving mechanical ventilation, but little is known about this syndrome in critically ill adults.To evaluate the validity and reliability of the WAT-1 in critically ill adults.A prospective, observational, open-cohort pilot study of critically ill adults receiving mechanical ventilation and regular administration of opioids for at least 72 hours. Patients were assessed for withdrawal twice daily on weekdays and once daily on weekends using the WAT-1 after an opioid weaning episode. The presence of iatrogenic withdrawal syndrome was evaluated once daily using modified (Fifth Edition) () criteria. All evaluations were blinded and performed independently. The criterion validity of the WAT-1 and the interrater reliability for WAT-1 and evaluations were determined.During 8 months, 52 adults (median age, 51.5 years) were enrolled. Eight patients (15%) had at least 1 positive assessment during their intensive care unit stay using the , compared with 19 patients (37%) using the WAT-1. The overall sensitivity of the WAT-1 was 50%, and its specificity was 65.9%. Agreement between WAT-1 and assessments was poor (κ = 0.102). The interrater reliability for the WAT-1 was 89.1% and for the was 90.1%.Despite showing reliability, the WAT-1 is not a valid tool for assessing the presence of iatrogenic withdrawal syndrome in adults.