Objective: To develop and test the validity and reliability of the Withdrawal Assessment Tool–1 for monitoring opioid and benzodiazepine withdrawal symptoms in pediatric patients. Design: Prospective psychometric evaluation. Pediatric critical care nurses assessed eligible at-risk pediatric patients for the presence of 19 withdrawal symptoms and rated the patient’s overall withdrawal intensity using a Numeric Rating Scale where zero indicated no withdrawal and 10 indicated worst possible withdrawal. The 19 symptoms were derived from the Opioid and Benzodiazepine Withdrawal Score, the literature and expert opinion. Setting: Two pediatric intensive care units in university-affiliated academic children’s hospitals. Patients: Eighty-three pediatric patients, median age 35 mos (interquartile range: 7 mos−10 yrs), recovering from acute respiratory failure who were being weaned from more than 5 days of continuous infusion or round-the-clock opioid and benzodiazepine administration. Interventions: Repeated observations during analgesia and sedative weaning. A total of 1040 withdrawal symptom assessments were completed, with a median (interquartile range) of 11 (6–16) per patient over 6.6 (4.8−11) days. Measurements and Main Results: Generalized linear modeling was used to analyze each symptom in relation to withdrawal intensity ratings, adjusted for site, subject, and age group. Symptoms with high redundancy or low levels of association with withdrawal intensity ratings were dropped, resulting in an 11-item (12-point) scale. Concurrent validity was indicated by high sensitivity (0.872) and specificity (0.880) for Withdrawal Assessment Tool–1 > 3 predicting Numeric Rating Scale > 4. Construct validity was supported by significant differences in drug exposure, length of treatment and weaning from sedation, length of mechanical ventilation and intensive care unit stay for patients with Withdrawal Assessment Tool–1 scores > 3 compared with those with lower scores. Conclusions: The Withdrawal Assessment Tool–1 shows excellent preliminary psychometric performance when used to assess clinically important withdrawal symptoms in the pediatric intensive care unit setting. Further psychometric evaluation in diverse at-risk groups is needed.