The incidence of thyroid microcarcinoma is rising due to the frequent use and improvement of fine-needle aspiration biopsy and ultrasonography. Since the recent update of the TNM (Tumor, Node, Metastasis) staging system for thyroid cancer, the importance of lymph node metastasis became more prominent. In the present study, we evaluated the prognostic factors and extension of thyroid microcarcinomas in Korean patients. The clinical and pathological findings in patients with thyroid microcarcinomas in a Korean hospital from January through December 2004 were evaluated. A total of 302 (50.2%) out of 601 cases of thyroid cancers were microcarcinomas. Evaluation of the histology revealed that nearly all of the cases (300 of 302) were of the papillary type. Analyzing patients of papillary thyroid microcarcinomas, 273 (91.0%) out of 300 patients of papillary microcarcinomas were women. Seventy-eight (26.0%) cases contained multiple tumor masses (> or =2), including 49 (16.3%) cases that were bilateral. There were 84 (28.0%) cases of extrathyroidal extensions and 89 cases (29.7%) of lymph node metastasis, but no cases of distant metastases. Application of the new staging system revealed 7 (2.3%) cases that changed from stage III to stage IVA. Thyroid microcarcinomas were also associated with poor prognostic factors and appear to exist at relatively higher cancer stages. Therefore, it is important to treat them as early and as vigorously as possible with extensive surgery, radioactive iodine therapy, and thyroxine suppression.