We have studied the thyroid tissue present in a cystic ovarian teratoma with the purpose of characterizing its structural and histochemical components. The methods of hematoxilyn and eosin, PAS, PAS/sialidase, Alcian blue at pH 2.5 and 1.0, were used, as well as, blocking reactions, Cason's thrichromic, and toluidine blue at pH 3.8. In the tissue we observed zones with follicles of normal appearance that varied considerably in size. The epithelium was constituted by cuboid cells surrounding a colloid substance of uniform aspect, that was strongly acidophilic, PAS positive and with variable reactivity with toluidine blue and weakly alcianophilic. With the thrichrome method, the different follicles stained in different tones. Parafollicular clear cells, bigger and less basophilic than follicular ones, with a nucleus having a lax chromatin, were identified. Cystic follicles were observed in whose cavities were conspicuous histyocytes surrounding rests of colloid substance, with metachromatic and PAS positive granules in their cytoplasm. Some groups of follicles were rudimentary, scarcely differentiated, and mostly without colloid. Others, with a greater degree of differentiation, showed diminutes follicles with intensely basophilic and PAS positive colloid. Beasides, follicular groups with excessive content of colloid appeared very distended and lined by a flat epithelium. According to the characteristics analyzed it may be concluded that: 1) it is a highly differentiated monoblastic teratoma, in which it appears non-encapsulated thyroid tissue; 2) the tumor resembles in different areas, either normal thyroid tissue, thyroid adenoma of the macrofollicular type, fetal or microfollicular and solid or embryonary adenoma, and follicles with the pattern of "thyroiditis".