A 43-year-old man presented with attacks of altered behaviour over a short period of time; they were associated with episodes of hypoglycaemia. The clinical suspicion of insulinoma prompted investigations that quickly established serum insulin and C-peptide levels to be elevated at the times when blood glucose values were low. A physical lesion was found in the head of the pancreas by means of computerised tomography and endo-duodenal ultrasound scan; an octreotide scan was negative. The patient underwent laparotomy and enucleation of a benign tumour, measuring 2.6 cm in diameter, lying within the head of the pancreas; histological examination confirmed it to be an insulinoma. Postoperatively, the patient's personality gradually became more normal and his fasting blood glucose concentrations returned to within normal limits. The diagnosis and management of insulinoma are discussed in the context of this clinical case; there is also reference to the protean clinical manifestations that may occur in this condition- and its differential diagnosis.