OBJECTIVES: The IMS CORE Diabetes Model (CDM) is an extensively validated simulation model designed for use in both for type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM) studies.  Validation to external studies is an important part of demonstrating model credibility, however, many studies are conducted over a relatively short period.  As the CDM is widely used to estimate long-term clinical outcomes in diabetes patients the objective of this study was to validate the CDM to contemporary outcomes data; including those with a 20-30 year time horizon. METHODS: A total of 81 validation simulations were performed stratified by duration of study follow-up (long-term defined as > 15 years follow-up); for long-term results simulation cohorts representing baseline DCCT and UKPDS cohorts were generated and intensive and conventional treatment arms were defined in the CDM.  Predicted versus observed macrovascular and microvascular complications and all cause mortality were assessed using the coefficient of determination (R2) goodness of fit measure. RESULTS: Across all validation studies the CDM simulations produced an R2 goodness of fit statistic of 0.90.  For validation studies with duration of follow-up ≤15 years the CDM achieved R2 values of 0.9 and 0.88 for T1DM and T2DM respectively.  In T1DM, validating to 30-year outcomes data resulted in an R2 of 0.72; for long-term 20-year validation to UKPDS in T2DM an R2 of 0.92 was obtained. CONCLUSIONS: This study supports the CDM as a credible tool for predicting the absolute number of clinical events in DCCT and UKPDS like populations.  With increasing incidence of diabetes worldwide this is of particular importance for healthcare decision-makers for whom the robust evaluation of alternative healthcare policies and therapeutic options is essential.