OBJECTIVES: Hereditary angioedema (HAE) is a rare but potentially life-threatening condition with intermittent and unpredictable oedema affecting the larynx, abdomen and extremities. This study aims to define the burden of HAE in England and Scotland as published data are limited. METHODS: A comprehensive, cross-sectional, retrospective study of the burden of HAE (type I and II) in England and Scotland. Three data collection methods: 1) secondary care data identified using Hospital Episode Statistics based on D.84.1 diagnostic code (defects in complement system, C1-esterase inhibitor deficiency); 2) primary care data accessed through The Health Improvement Network (THIN) database using C-376-000 HAE diagnostic code. Both database analyses identified patients diagnosed ≤10 years and include all episodes ≤2years.  Costs were calculated using most up-to-date best-matched HRG tariffs. 3) Primary research in five secondary care centres in England and Scotland collecting information on >100 patients (>18 years) via medical records ≤2years, matched with patient self-completion questionnaires and centre interviews. Selected centres represent different approaches to HAE management to ensure a national representative sample. RESULTS: Data collection from all three phases of the study is on-going. Early results from 1) indicate 1,174 HAE patients admitted to hospitals in England, for any reason, in the past two years. Mean length of stay, including day cases, was 2.8 days. The annual total cost of secondary care in England was £3,227,149, corresponding to per HAE patient cost of £2,749. CONCLUSIONS: This is the first comprehensive UK HAE cost of illness study, providing a comprehensive understanding from both NHS and patient perspectives. Important insight into patient demographics, pathway of care, treatment patterns, and any regional or sub-population differences in the standard of care will be provided, helping to raise disease awareness in the UK.