A 56-year-old patient was admitted to the Department due to recurrence of symptoms: sudden weakness, dizziness, excessive sweating, palpitations, which cleared up after food intake. Physical examinations and initial diagnostic tests did not reveal any abnormalities. Abdominal ultrasonography and magnetic resonance showed a heterogenous tumour of the pancreas with features of insulinoma. The prolonged supervised fast test that was applied induced hypoglycaemic symptoms. The level of glucose and insulin was at the lower range of the fast test. The tumor was surgically removed, and the suspicion of insulinoma was confirmed by histopathologic examination.