A 59-year-old diabetic patient was admitted with loss of consciousness and convulsions. Hypercalcaemia and hypoglycaemia were discovered. Computed tomographic and technetium scans revealed a retrosternal paramedian nodule. Radical thymectomy was performed by median sternotomy. Diagnosis of intrathymic parathyroid adenoma was made. One year later the patient had good diabetes control without clinical symptoms.