BACKGROUND: Phase of Illness is used to describe the stages of a patient's illness in the palliative care setting. Categorization is based on individual needs, family circumstances, and the adequacy of a care plan. Substantial (? = .67) and moderate (? = .52) inter-rater reliability is demonstrated when categorizing adults; however, there is a lack of similar studies in pediatrics. OBJECTIVE: To test the inter-rater reliability of health-care professionals when assigning pediatric palliative care patients to a Phase of Illness. Furthermore, to obtain user views on phase definitions, ease of assignment, feasibility and acceptability of use. METHOD: A prospective cohort study in which up to 9 health-care professionals' independently allocated 80 pediatric patients to a Phase of Illness and reported on their experiences. This study took place between June and November 2017. RESULTS: Professionals achieved a moderate level of agreement (? = 0.50). Kappa values per phase were as follows: stable = 0.63 (substantial), unstable = 0.26 (fair), deteriorating = 0.45 (moderate), and dying = 0.43 (moderate). For the majority of allocations, professionals report that the phase definitions described patients very well (76.1%), and they found it easy to assign patients (73.5%). However, the unstable phase caused the most uncertainty. CONCLUSION: The results of this study suggest Phase of Illness is a moderately reliable, acceptable, and feasible tool for use in pediatric palliative care. Current results are similar to those found in some adult studies. However, in a quarter of cases, users report some uncertainty in the application of the tool, and further study is warranted to explore whether suggested refinements improve its psychometric properties.