Aims The aim of the study was to evaluate registered children’s nurses’ approach to the assessment and management of withdrawal syndrome in children. Background 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 standardise 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. Design Cognitive interviews using with clinical vignettes. Methods Interviews were undertaken with 12 nurses to explore the cognitive processes they utilised when assessing children using the Sedation Withdrawal Score (SWS) tool. Interviews took place in Autumn 2013. Findings 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 SWS 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. Conclusion Cognitive interviews with vignettes provided insight into nurses’ judgement and decision-making. The SWS does not standardise the assessment of withdrawal due to the complexity of the context within which assessments take place and the difficulties of determining the cause of equivocal behaviours in children recovering from critical illness.