OBJECTIVE: To assess the natural history of ultrasonographically diagnosed benign ovarian teratomas in asymptomatic women. METHODS: We conducted a retrospective observational cohort study of 408 women (mean age 36.6 years, range 14-81 years) diagnosed as having an ovarian teratoma by transvaginal ultrasonography (except eight who only had a transabdominal study done) between January 2003 and December 2013 at a single tertiary care institution. Six hundred thirteen women were diagnosed with ovarian teratoma of whom 205 were promptly treated surgically, leaving 408 patients followed conservatively with follow-up scans at 3 and 6 months from diagnosis and then yearly. The ultrasonographic diagnosis of a benign ovarian teratoma required at least one of the following features: a cystic mass with mixed echogenicity, thick band-like echoes, a fat-fluid level, or echogenic tubercle with posterior shadowing. Clinical, ultrasonographic, and histologic data (in case of surgery) were retrieved for analysis. RESULTS: During follow-up, 130 of 408 (31.8%) women underwent surgery. The main reason for surgery was the physician's recommendation according to our protocol (n=115). One patient had adnexal torsion. Most surgeries (112/130 [86.2%]) were performed within the first 5 years after diagnosis. The remainder (278/408) is still being followed (median time 45.6 months, range 6-147 months). The vast majority of these lesions had no change and women remain asymptomatic. Histologic diagnosis of tumors removed surgically revealed a benign ovarian teratoma in 103 of 130 (79.2%) of the women. There were two borderline tumors, four endometriomas, three fibromas, seven serous cysts, two mucinous cysts, two stroma ovarii, seven other benign, and no case of malignant tumor. CONCLUSION: Our results demonstrate that expectant management might be a reasonable option for managing asymptomatic women who receive a ultrasonographic diagnosis of a benign ovarian teratoma. The risk of undergoing surgery for this lesion decreases significantly after 5 years to follow-up. With careful observation, the risk of missing a diagnosis of malignancy is low.