The aim of the study was to evaluate registered children's nurses' approaches to the assessment and management of withdrawal syndrome in children. Assessment of withdrawal syndrome is undertaken following critical illness when the child's condition may be unstable with competing differential diagnoses. Assessment tools aim to standardize and improve recognition of withdrawal syndrome. Making the right decisions in complex clinical situations requires a degree of mental effort and it is not known how nurses make decisions when undertaking withdrawal assessments. Cognitive interviews with clinical vignettes. Interviews were undertaken with 12 nurses to explore the cognitive processes they used when assessing children using the Sedation Withdrawal Score (SWS) tool. Interviews took place in Autumn 2013. Each stage of decision-making-noticing, interpreting and responding-presented cognitive challenges for nurses. When defining withdrawal behaviours nurses tended to blur the boundaries between Sedation Withdrawal Score signs. Challenges in interpreting behaviours arose from not knowing if the patient's behaviour was a result of withdrawal or other co-morbidities. Nurses gave a range of diagnoses when interpreting the vignettes, despite being provided with identical information. Treatment responses corresponded to definite withdrawal diagnoses, but varied when nurses were unsure of the diagnosis. Cognitive interviews with vignettes provided insight into nurses' judgement and decision-making. The SWS does not standardize the assessment of withdrawal due to the complexity of the context where assessments take place and the difficulties of determining the cause of equivocal behaviours in children recovering from critical illness.