A 53- year-old woman without a previous history of thyroid disease was scheduled for mastectomy. On arrival in the operating theatre unpremedicated she appeared restless and tachycardic. Midazolam and fentanyl was administered intravenously. Concomitantly, sinus tachycardia developed and a flush reaction was observed in the skin of the thoracic region and neck. The blood pressure increased to 265/160 mmHg and the patient lost consciousness and became apnoeic. Unconsciousness and apnoea lasted for approximately 25 min and the operation was postponed. Further investigations revealed an elevated serum free thyroxine level and suppressed serum thyrotropin diagnostic of hyperthyroidism. The serum TSH receptor antibody concentration was elevated, indicating that the patient was suffering from Graves' disease. We present a case of a previously unknown hyperthyroid patient, with breast cancer, presenting as a thyroid crisis on induction of anaesthesia. Although being quite a rare occurrence, unsuspected thyroid disease should be borne in mind when an agitated patient enters the operating theatre.