PURPOSE: Occult papillary thyroid carcinomas (OPC) are defined as tumors measuring <15 mm. These tumors are believed to be a less aggressive subset of papillary cancers. They generally behave more like benign lesions and are often more conservatively treated. However, it is unclear if a cancer 1.0 to 1.5 cm in diameter will have a similar favorable clinical behavior as tumors <1.0 cm (micropapillary thyroid carcinoma). Therefore, a retrospective chart review study of patients with OPC in order was carried out in order to answer this question and characterize the biology and optimal treatment for OPCs. METHODS: From October 2001 to January 2007, Among the impalpable thyroid nodules detected incidentally during screening examinations, 260 patients underwent surgery for occult papillary thyroid cancer (OPC) at Kangnam Cha University hospital. The data from these patients was analyzed retrospectively. The mean follow up period was 25.6 +/- 14.5 (max: 63, min: 1) months. RESULTS: The mean age of these patients was 42.8 years, and 233 (89.5%) were female. 46.2% of patients underwent a total or neartotal thyroidectomy, and 54.6% underwent a central lymph node dissection. Of the 260 patients, 55 (21.2%) had lymph node metastases. The OPC presented with signs of aggressiveness including multifocality (34.2%), bilaterality (17.7%), capsular invasion (52.7%), and lymph node metastases (21.2%). A progressively increasing frequency of the signs of tumor aggressiveness was observed with increasing tumor size at presentation. LN metastases were associated with the tumor size (P=0.0063), extracapsular invasion (P=0.0015) and multfocallity (P=0.0020). However, there was no association with age and gender. With a follow-up of up to 63 months, 3 patients had a local recurrence (0.014%). No patients currently have active disease and no patients with OPC died during this period. CONCLUSION: In OPC patients, there is a progressively increasing frequency of the signs of tumor aggressiveness with increasing tumor size. Moreover, a small size itself cannot guarantee low risk and low recurrence rate. The prevalence of LN metastases and extracapsular invasion were higher in those with a tumor size >0.5 cm. A near-total or total thyroidectomy with a central lymph node dissection is the preferred treatment. The early detection and treatment of OPC might be warranted through the routine use of thyroid USG and USG-guided FNA.