The detailed case histories of 5 patients with hypoglycaemic episodes secondary to islet cell adenoma of the pancreas are presented. Clinical recognition of this syndrome remains the major problem but a full and detailed medical history is usually strongly suggestive of the correct diagnosis. The clinical diagnosis is confirmed by the repeated demonstrated that: 1) Symptomatic episodes produced by fasting are in fact due to hypoglycaemia (plasma glucose level less than 2.5 mmol/litre) 2) Such episodes are relieved by glucose administration 3) Concomitant hyperinsulinaemia is present (serum insulin greater than 8 micro-units/ml in the fasting state). Surgical resection of the adenoma produces a complete cure but the identification of the lesion at operation may be difficult and preoperative means of accurate localisation may be needed.