A simple cost-utility model was developed to evaluate new interventions for Type 1 diabetes mellitus by assessing the association between the interventions' effects on mean glycated haemoglobin and long-term complications and the risk of hypoglycaemic events. High-quality, recently reported data specific to people with Type 1 diabetes mellitus were identified by a systematic review. Methods: Through a systematic review, we identified complications associated with Type 1 DM and data describing the long-term incidence of these complications. An individual patient simulation model was developed and included the following complications: cardiovascular disease, peripheral neuropathy, microalbuminuria, end-stage renal disease, proliferative retinopathy, ketoacidosis, cataract, and adverse birth outcomes. Risk equations were developed from published cumulative incidence data and hazard ratios for the effect of glycated haemoglobin, age, and duration of diabetes. We validated the model by comparing model predictions with observed outcomes from studies used to build the model (internal validation) and from other published data (external validation). We performed illustrative analyses for typical patient cohorts and a hypothetical intervention.Results: Model predictions were within 2% of expected values in the internal validation and within 8% of observed values in the external validation (percentages represent absolute differences in the cumulative incidence). Conclusions:The model utilised high-quality, recent data specific to people with Type 1 DM. In the model validation, results deviated less than 8% from expected values.