목적: Papillary thyroid carcinomas frequently occur as two or more separate foci within the thyroid gland (18~87 %). However, those multifocal tumors are easy to be undetected by preoperative radiologic evaluations, which lead to remnant disease after initial surgery. In addition, there are no known predictors. We aimed to study the incidence of multifocal PTMCs, predictive factors, the chance of bilateral tumors and diagnostic accuracy of preoperative radiologic evaluation. 방법:Two hundred and seventy-seven patients with PTMC were included in this study. All patients underwent total thyroidectomy as initial treatment. Medical records, pathology reports, radiological reports were reviewed for analysis. 결과:Multifocal PTMCs were detected in 100 (36.1%) of 277 patients. The mean number of tumors in each patient was 1.6 1.1, ranging from 1 to 10. The additional tumor foci were significantly smaller than the largest primary tumors (P <0.001). There was no significant relationship between primary tumor size and the presence of contralateral tumors. With more tumors detected in dominant lobe, there was greater chance of contralateral tumors 18.8% with single tumor focus, 30.2% with 2 tumor foci, and 46.2% with 3 or more tumor foci in dominant lobe. Sensitivity of preoperative sonography were 42.7% for multifocal tumors and 49.0% for bilateral tumors. With multivariate analysis, nodular hyperplasia was the only significant factor for multifocal tumors. 결론:In cases of PTMCs, the incidence of multifocal tumors are high. However, additional tumor foci are too small to be diagnosed preoperatively, especially under present policy on radiologic screening tests for PTC. The size of a primary PTMC is not associated with multifocal or bilateral tumors. However, multifocal PTMCs has high risk of bilateral tumors, necessitating more extensive surgery or more thorough follow-up. Further investigation is needed to clarify the predictive factors.