OBJECTIVES: Adherence with 5-aminosalicylic acid (5-ASA) treatments has been shown to be associated with a reduction in disease relapses in UC patients. The aim of this budget impact analysis was to explore and quantify how adherence with individual 5-ASA treatments may impact direct medical costs, through prevented relapses, in the UK. METHODS: A 1-year decision analytic budget impact model was developed to combine data from a UK-based adherence study of 5-ASA treatments with a chart review study on UC costs by relapse status in the UK. The model calculates the rates of disease relapses, remissions, and associated costs, based on adherence rates of each 5-ASA medication. The model also allows running simulations of relative changes in treatment utilization to show the associated budget impact from the perspective of the National Health Service (NHS). RESULTS: Higher adherence rates (48.3% for MMX Multi-Matrix System® [MMX] mesalamine; 40.7% for delayed release mesalamine [DRM] 800mg; 36.7% for modified release mesalamine [MRM]; 31.8% for controlled release mesalamine [CRM] 1000mg; 29.7% for controlled release mesalamine [CRM] 500mg; 29.6% for delayed release mesalamine [DRM] 400mg; 27.8% for balsalazide) were associated with lower hospitalization rates (6.6%; 7.3%; 7.7%; 8.1%; 8.3%; 8.3%; and 8.5%, respectively), lower annual hospitalization costs (£330; £365; £383; £404; £414; £414; and £422, respectively), and lower other medical costs, excluding 5-ASAs (£282; £292; £298; £305; £307; £308; and £310, respectively).  The model showed that a hypothetical move from the current utilization mix of 5-ASA treatments to the 5-ASA with the highest adherence rate could save the NHS approximately £92,800 annually per 1,000 UC patients. CONCLUSIONS: As non-adherence in UC is associated with costly medical resource utilization, significant cost-offsets could be achieved within the NHS by favoring the 5-ASA treatment with the highest adherence rate.