Background: Much of the literature on the healthcare transition from pediatric to adult centered-care focuses on how to prepare adolescents and young adults with chronic illness for a successful healthcare transition, with much less emphasis on how to measure whether the healthcare transition was successful. Research on healthcare transition outcomes is fraught with methodological difficulties, making it difficult to generalize and compare findings across studies. To address these gaps, we began an integrative research agenda to develop a multidimensional, multi-informant (i.e., young adult, parent, and healthcare provider versions) measure of healthcare transition outcomes, the Healthcare Transition Outcomes Inventory (HCTOI). Our preliminary qualitative study defined six content domains for the measure. The current study describes the development and refinement of the HCTOI item pool. Results: Using rigorous measurement development methods including mixed qualitative and quantitative methods, the researchers developed a pool of 88 items evaluating the extent to which a young adult with T1D is successful on each of the six domains of healthcare transition outcomes. Ten physician and psychologist experts provided content validity ratings and feedback and the item pool was reduced and refined to 69 items. Cognitive interviews were then conducted on every item with end users (young adults, parents, and healthcare providers) and the item pool was further reduced to 54-item young adult- and parent-versions and a 47-item healthcare provider version. Conclusions: Each stage informed development and refinement of the item pool. The HCTOI represents the first multi-informant, rigorously developed item pool that comprehensively measures the multiple components of the transition from pediatric to adult specialty healthcare. Discussion includes clinical implications and next steps for validation, item reduction and refinement. Disclosure J. Pierce: None. T. Wysocki: None. A. Gannon: None.