A patient with hyperthyroid Graves' disease and struma ovarii is described. She had pre-existing Graves' disease and positive thyrotrophin receptor antibody. She was treated with radioactive iodine 5 months before she became pregnant. Paripartum she had torsion of an ovarian cyst with histological evidence of a functional struma ovarii. Immediate exacerbation of her thyrotoxic state was observed after operation as a result of release of thyroid hormone from the tumour. It is postulated that the tumour was stimulated by circulating thyrotrophin receptor antibody.