Aim The risks of fasting during Ramadan in patients with adrenal insufficiency are unknown. The aims of this study were to evaluate these risks in such patients, to determine the risk factors and finally to set some recommendations. Methods It is a cross-sectional study about 180 patients with known and treated adrenal insufficiency. The patients responded to a 14-item questionnaire concerning their knowledge about the disease and fasting during the last month of Ramadan. Results There were 132 women and 48 men. The mean age was 47.6 +/- 15.0 years (14-79). One hundred and thirty eight patients (76.7 %) were advised by their physician not to fast. Ninety-one patients (50.5 %) tried to fast. Complications occurred in 61 cases (67.0 %): asthenia in 88.5% of cases, intense thirst in 32.8 %, symptoms of dehydration in 49.2% and symptoms of hypoglycaemia in 18 %. One patient was hospitalized. Fifty-five patients (60.4 %) were able to fast for the whole month. Age, gender, duration of the disease, its primary origin, associated hypothyroidism, diabetes mellitus, hypertension or diabetes insipidus and daily dose of hydrocortisone did not significantly differ between fasters and non-fasters, full-month-fasters and partial-month-fasters, and fasters with complications and fasters without complications. The frequency of adequate knowledge about the disease was significantly higher in full-month-fasters vs. partial-month-fasters, and in fasters without complications vs. those with complications. Conclusion In patients with adrenal insufficiency, fasting can cause complications especially if the level of knowledge about the disease is low.