During the last 15 years in the Pathology Laboratory of Aretaieion University Hospital, 256 cases of teratomas were examined and 11 cases (4.3%) presented characteristics of struma ovarii. Eight patients aged 19-34 years and four 41-74 years (median 41 years). One 74-year-old patient presented with virilization signs but pain and a pelvic mass were the most frequent symptoms. The tumors were unilateral in 9/11 cases, the size ranged from 5-17 cm, and they were compact or microcystic in 10/11 cases and cystic in one case. Histological pattern was microfollicular in 5/11 cases, solid in 3/11 cases, pseudoglandular in 2/11 cases, with focal clear and oxyphil cell changes, and infiltration of the fibrous wall in one case. Tumorectomy was performed in 9/11 cases and total hysterectomy with adnexa in two cases with large tumors (16-17 cm in diameter). Immunohistochemistry is useful in the recognition of struma in atypical cases. No malignant struma ovarii was observed in our cases. Tumorectomy is the appropriate therapeutic approach because of the benign nature of these tumors despite atypical histological features and the young age of the patients.