Three cases of cystic struma ovarii in women aged 16, 20 and 40 are described. All patients had an asymptomatic ovarian mass at ultrasound scan. The tumours, all of which were unilateral and confined to the ovary, ranged from 7 to 10 cm in the greatest dimension. Two lesions were unilocular, the third multilocular, and all were filled with green fluid. Microscopic examination showed cysts with fibrous wall lined by non-specific-appearing epithelial cells. In the wall of the cysts, there was a small number of thyroid follicles. In one case, an association with a cystic mature teratoma was seen. The paucity of thyroid follicles and the non-specific appearance of the epithelial cells required a careful sampling and immunohistochemical staining for thyroglobulin to establish an exact diagnosis. The postoperative period was uneventful and thyroid function remained normal. In conclusion, cystic struma is probably often underdiagnosed and should be considered when evaluating cystic ovarian tumours whose features are not obviously those of another tumour type. A careful search for thyroid follicles should be undertaken. In problematic cases immunohistochemical staining for thyroglobulin may be required.