OBJECTIVES: The largest proportion of costs in diabetes is due to complications of the disease. Hypoglycaemia is a common, chronic complication of drug treatment in diabetes. The aim of this study was to survey the cost of a major hypoglycaemic event. METHODS: A systematic literature review was carried out on all relevant assessments found in published literature. A study was regarded relevant if it included the cost of a major hypoglycaemic event. RESULTS: Relevant studies were found in the following countries: Canada, Germany, Sweden, Switzerland, UK and US. Five studies collected primary data and six studies based the cost of a hypoglycaemic event on assumptions around treatment patterns. A major hypoglycaemic event was defined differently in the different studies. Four studies defined the state as requiring third-party medical intervention which means assistance from health care services with costs ranging from €293 to €586. The state was also defined as requiring third party assistance by medical and/or family assistance in four studies, these estimates ranged from €190 to €1643. One study defined the state as requiring assistance from another person excluding medical intervention (€69), while two studies defined the state from the ICD-9-CM codes, which requires a visit to a health care provider and presented costs at €950 and €4083, respectively. The difference in costs between the studies can be explained by different health care systems, whether direct and/or indirect costs were included and whether hospitalisation was excluded. Indirect costs were included in three studies. CONCLUSIONS: Which costs to include in a study is determined by which definition of a major hypoglycaemic event that is chosen. For this reason it is important that health economic models apply matched definitions to the clinical studies they are modelling.