OBJECTIVES: Hypoglycaemia is common in diabetic patients, often as a side effect of a treatment.  Its occurrence can influence patients’ professional life and generate extra direct costs to the healthcare system. Severe (requiring another person’s assistance) hypoglycaemic events (SHEs), especially leading to hospitalisation, are associated with the highest burden and for that reason their real frequency and costs should be calculated properly. The aim of the study was to assess the accuracy of reporting of hypoglycaemia to the National Health Fund in Poland.  METHODS: A combination of prospective and retrospective designs was applied. A detailed retrospective analysis of 117 hospitalised patients’ case histories was performed from the period of February-March 2013.  In addition, prospectively, in March and April 2013, 45 patients were interviewed and their clinical records were analysed. Information was collected in special case report forms. Real hospitalisation costs were also calculated.  RESULTS: In the retrospective arm hypoglycaemia affected 42 patients (55% of type 1 and 25% of type 2) and 85 SHEs were recorded. Only 2 patients were reported in the hypoglycaemia DRG group compared to a broader diabetes/hyperglycaemia DRG group, where the financing level is higher. A financial difference equal to 7,082 EUR was generated due to more favourable classification of these cases. In the prospective arm 20 patients experienced hypoglycaemia, whereas only one was coded as hypoglycaemic. It has generated 2,247 EUR difference. Even with this financially more favourable, (still clinically justified but less precise) coding hypoglycaemic patients generated losses equal to 23,628 EUR in retrospective and 5,053 EUR in prospective group.  CONCLUSIONS: Our study provided a direct proof of underreporting of hypoglycaemia in clinical hospital settings in Poland caused by DRG- based financing standards. There is a need of reflecting real treatment cost in the level of reimbursement in this group of patients.